In cognitive psychology, a nonsense syllable is a word-like string of letters that is not intended to have any established meaning; it is a special case of a non-lexical vocable. Nonsense syllables have been extensively used in experimental psychology, especially the psychology of learning and memory.
Nonsense syllables were first introduced by Hermann Ebbinghaus in his experiments on the learning of lists. His intention was that they would form a standard stimulus so that experiments would be reproducible. However, with increasing use it became apparent that different nonsense syllables were learned at very different rates, even when they had the same superficial structure. Glaze introduced the concept of association value to describe these differences, which turned out to be reliable between people and situations. Since Glaze's time, experiments using nonsense syllables typically control association value in order to reduce variability in results between stimuli.
Nonsense syllables can vary in structure. The most used are the so-called CVC syllables, composed of a consonant, a vowel, and a consonant; these have the advantage that they are all pronouncable (or nearly all, depending on how you feel about pronouncing, say, "XUQ"). They are often described as "CVC trigrams", reflecting their three-letter structure. Obviously many other structures are possible, and can be described on the same principles, e.g. VC, VCV, CVCV. But the CVC trigrams have been studied most intensively; for example, Glaze determined association values for 2019 of them.
The term nonsense syllable is widely used to describe non-lexical vocables used in music, most notably in scat singing but also in many other forms of vocal music. Although such usages do not invoke the technical issues about structure and associability that are of concern in psychology, the essential meaning of the term is the same.