Obviate (abbreviated obv) person deixis is a grammatical person marking that distinguishes a non-salient (obviative) referent from a more salient (proximate) referent in a given discourse context. The third person proximate is sometimes referred to as the "fourth person." 
Obviate/proximate distinctions are common in some indigenous language families in northern North America. Algonquian languages are perhaps best-known for obviation, but the feature also occurs in some Salishan languages, as well as in the more southern Keresan languages. Obviation in the Kutenai language, a North American language usually described as a language isolate, is notable because of the language's proposed relationships with the Salishan and Algonquian families.
The following is a typical example of obviate/proximate morphology in the Eastern dialect of the Algonquian Ojibwe language, in which the obviative is marked on nouns and demonstratives and reflected in pronominal verb affixes:
Maaba dash shkinwe wgii-bwaadaa wii-bi-yaanid myagi-nishnaaben waa-bi-nsigwaajin maaba dash oshkinawe o-gii-bawaad-am-n wii-bi-ayaa-ini-d mayagi-nishanaabe-an x-wii-bi-nis-igo-waa-d-in this EMP young.man 3-PAST-dream-3INAM-OBV FUT-coming-be.at-OBV-3 foreign-people-OBV REL-FUT-coming-kill-INV-3-OBV
'Then this (PROX) young man (PROX) dreamed (PROX) that foreigners (OBV) would come (OBV) to kill (OBV) them (PROX).'
Note that this example shows that the proximate referent need not necessarily be the subject of a clause.
The Algonquian Potawatomi language is notable for having two "degrees" of obviation. As is seen in the following example, a "further obviative" referent deemed even less salient than the obviative referent can be marked by reduplication of the obviative suffix:
waposo waposo-n waposo-n-un rabbit rabbit-OBV rabbit-OBV-OBV /proximate/ /obviative/ /further obviative/ 
Obviation in the Ingush language, a heavily dependent-marking language, is an exception to the generalization that the obviative occurs in head-marking languages. Obviation is not overtly marked in Ingush, but is implied by the fact that certain constructions are only possible when one referent has salience over another.
For example, if a non-subject-referent has salience over the subject and precedes the other co-referent, reflexivization (normally used only when there is a coreferent to the subject) is possible. This is shown in the example below where the non-subject-referent appears to have salience over the subject:
Muusaajna shii zhwalii t'y-weaxar Musa-DAT 3S-RFL-GEN dog on-bark-WITNESSED PAST
'Musa's dog barked at him.'
If the subject is salient ("proximate"), on the other hand, the subject's possessor may not antecede the third person object, and the possession must be indirectly implicated as follows:
Muusaa siesaguo liex Musa wife-ERG seek
'Musa's wife is looking for him.' (Lit. 'The wife is looking for Musa.')