The Watershed is a term used to describe a time in television schedules which divides the period when it is permissible to show television programmes which have 'adult content' from the period when it is not. Adult content can be generally defined as having nudity, explicit sexual intercourse, graphic violence, strong language, or drug references or use.
According to COMFER, the watershed starts at 10:00 p.m., and finishes at 8:00 a.m. the next morning.
On Australian television MA15+ programmes are not allowed to be shown before 9:00 p.m. at night. However, the time difference between New South Wales and Queensland during daylight saving in the former, and the fact that some stations that broadcast into southern Queensland from northern New South Wales (specifically the Gold Coast), effectively pushed the watershed time back to 8:30 p.m. Complaints from concerned citizens forced these stations to broadcast as per local time.
AV15+ is a television specific variation of the MA15+ rating indicating the program contains "Adult Violence" and programs given this rating are not allowed to be screened before 9:30PM. As such, most MA15+ programs generally begin after 9:30pm.
Subscription narrowcast channels may broadcast R18+ materials, but must ensure that the material is restricted to access by those with appropriate disabling devices.
Since its inception in 1995 Pay TV channel Adults Only has only broadcast from 10:45 p.m. (9:45 p.m. at times) until 4:45 a.m. Although it is advertised until 6:00 a.m., it broadcasts only a test signal in the intervening time. However since the inception of Adults Only Select (a pay-per-view service) in 2004, it has all but eliminated the watershed time.
In the main German-speaking countries, the watershed is between 10:00 p.m. and 5:30 a.m. However, programmes marked "Keine Jugendfreigabe" (not approved for minors) under FSK may only be shown after 11:00 p.m.
According to the new rules of DJCTQ, TV shows rated for people over 16 years old can only air between 10 p.m. and 6 a.m., while shows rated for those over 18, can only air between 11 p.m. and 6 a.m. The ratings, however, are administered by the television networks themselves, which are responsible for rating their shows, which means that DJCTQ acts merely as content controler, acting after the inappropriate content for children has already been aired (as it happened with Rede Globo's Duas Caras). Self-censorship is not uncommon, such as when the head of Rede Record edited all nude scenes in the telenovela Poder Paralelo, which could air such scenes, once it is broadcast after 10 p.m. This generated controversy, since violent scenes remained untouched.
The watershed starts at 9:00 p.m., and finishes at 6:00 a.m. the next morning. Sexually explicit content, some offensive language and other adult material is not allowed outside the watershed.
See also: Censorship in Canada
In Finland, all the major television companies (YLE, MTV3 and Nelonen Media) have agreed not to show 11 rated content before 17:00, 15 rated content before 21:00 and 18 rated content before 23:00. 15 and 18 rated programs are marked on the schedule with K15 or K18 respectively but 11 rated content is not marked. In 2008 a K13 rating was introduced. Content with a K13 rating is not shown before 19:00. Television channels use their own discretion to decide the ratings. However non-sexual nudity itself is not grounds for a mature rating.
Greek TV uses a triple-tier watershed, along with a five-tier color-coded decal scheme, displayed in the beginning and in regular intervals during all broadcasts except for news bulletins. A white rhombus in green indicates programming that is suitable for all ages and times of day. A white circle in blue ("Suitable for children, but parental consent is desired") is used in cases that may potentially upset children. A white triangle in orange ("Suitable for minors, but parental consent is necessary") indicates mild violence or language that will upset younger children, and such programming is only allowed between 7:00 p.m. and 6:00 a.m. A white square in purple ("Unsuitable for viewers under the age of 15") indicates foul language, violence, or erotic -but not explicit- situations and such programming is only allowed between 9:00 p.m. and 6:00 a.m. A white X in red ("Adults only") indicates programming allowed only between midnight and 6:00 a.m. Foul language before midnight is punishable by fine, except when used in the context of a suitably labelled movie film, theatrical play, etc. The color-coded ratings are mandatorily displayed and verbally announced at the beginning of each broadcast. These provisions are enforced by the National Radio and Television Council (ESR), an independent authority, the executive members of which are appointed by the leaders of all parliamentary parties, preferably by unanimous consent and in extremis by an 80% supermajority.
The watershed in Ireland starts at 9.00 p.m. and finishes at 5.30 a.m. the next morning. On premium film or pay-per-view services requiring a subscription, the watershed starts at 8.00 p.m. However, 12, 15 and 18 rated films can be shown on PIN protected channels (such as Sky Movies) at any time of the day. Viewers are required to enter their PIN to view.
See also: Media in Ireland
According to Codice Tv e minori (Code for Children and Television, 2002), all the channels must broadcast "general audience" programmes from 7 a.m. to 10:30 p.m., and programmes "directed to children and adolescents" only from 4 p.m. to 7 p.m.. After 10:30 p.m. +14 programmes can be aired. +18 programmes cannot be broadcasted on television anytime.
On free-to-air channels (but not SKY) programming rated Adults Only is only allowed between 8:30 p.m. and 5:00 a.m. Adults Only programming is also allowed to air between midday and 3:00 p.m. on school days (this situation is probably unique to New Zealand). Sports programming and news bulletins are exempt from the system altogether, but do still carry warnings before certain stories.
Open channel terrestrial TV stations (RTP, SIC and TVI) can only broadcast programs and movies rated 16 or 18 between 11pm and 6am. On cable TV however there are no restrictions of broadcasting, except pornography which can't be broadcasted at all without encrypted signal, requiring an IRD to be seen.
Switzerland has no watershed. However, broadcasters are required by law to avoid any confrontation of minors with unsuitable programming through the choice of transmission time. 
According to Ofcom, the watershed on standard television in the UK starts at 9:00 p.m., and finishes at 5:30 a.m. the next morning. Programmes that are 15+ are shown during this period. However, some 12+ shows can be shown before 9:00 p.m., such as The Simpsons, Malcolm in the Middle and Doctor Who. On premium film or pay-per-view services requiring a subscription, the watershed starts at 8:00 p.m. However, 12, 15 and 18 rated films can be shown on PIN protected channels (such as Sky Movies) at any time of the day. Viewers are required to enter their PIN to view. There should be a gentle transition to adult material, and 18-rated material is not allowed to be shown before 9:00 p.m.
See also for the UK: The Ofcom Broadcasting Code - Section 1
The term "watershed" is not used in this context in the United States. In the US, the "safe harbor" for "indecent" programming begins at 10:00 p.m. and ends at 6:00 a.m. the next morning (all time zones). However, content that is considered "obscene" (including explicit human sexual intercourse) is never allowed by the FCC rules for broadcast stations. Those content rules only apply to channels broadcast terrestrially and not those only available on cable. Consequently, restricted-access networks (like the premium channels HBO and Showtime and adult channels Playboy TV and Spice) have taken advantage of considerably more leeway in their programming.
Some American television scenes famous for "pushing the envelope" (such as limited nudity on NYPD Blue) were aired in the 10:00 p.m.–11:00 p.m. hour; however, these broadcasts were before the Safe Harbor in the Central and Mountain time zones, where programming scheduled for 10:00 p.m. EST would typically be broadcast starting at 9:00 p.m. (using a one-hour delay in Mountain time).
Because the FCC interprets safe harbor times as local time, some network affiliates can be fined for airing an "indecent" program simultaneously with another station whose broadcast is officially at a later time and wouldn't be fined. Such was the case with CBS, whose affiliates were fined US$3.63 million for a repeat of the episode "Our Sons and Daughters" of Without a Trace in December 2004. The program was flagged for depicting an orgy involving teenagers. It was televised at 10:00 p.m. in the Eastern and Pacific time zones (within the watershed), but at 9:00 p.m. in Central and Mountain times (outside the watershed). The FCC split its fine among the 111 CBS affiliates covering these time zones. It is still being appealed by those stations.
In the 1970s, the ill-fated Family Viewing Hour tried to make the 8:00 p.m.-9:00 p.m. hour (7-8:00 p.m. Central/Mountain) safe for family consumption, but was overturned in court due to the way it was instituted.
Today, television can show a number of programmes. Usually the content of these programmes is rated, by a number of people (usually called advisory board). Some of the programmes may be suitable for children (of different ages), other content may not be suitable for children. The watershed is the time of the day where content not suitable for children may be shown. This is usually during the night (starting in the later evening, and ending in the early morning)
The content that is not suitable for children to see usually includes sex or nudity outside a functional context. People bathing naked in a lake may be ok, people doing a striptease in a night club or cabaret may not be. Other content generally not suitable includes various levels of violence or the use of illegal drugs. Sometimes strong language (full of expletives) is also seen as unfit for children.