In linguistics and especially phonology, functional load, or phonemic load, is the collection of words that contain a certain pronunciation feature (a phoneme) that makes distinctions between other words. Phonemes with a high functional load distinguish a large number of words from other words, and phonemes with a low functional load distinguish relatively fewer words from other words. The omission or mishearing of features with a high functional load thus leads to more confusion than features with a low functional load.
Overview
The term "functional load" goes back to the days of the Prague School; references to it can be found in the work of Vilém Mathesius in 1929. Its most vocal advocate was André Martinet, a historical linguist who claimed it was a factor in the likelihood of a phonological merger.
The first suggested measurement for functional load was the number of minimal pairs, but that does not take into account word frequency and is difficult to generalize beyond binary phonemic oppositions. Charles Hockett proposed an information theoretic definition in 1955, which has since been generalized. Now, with a large text corpus, one can compute the functional load of any phonological contrast including distinctive features, suprasegmentals, and distinctions between groups of phonemes. For instance, the functional load of tones in Mandarin Chinese is as high as that of vowels: the information lost when all tones sound alike is as much as that lost when all vowels sound alike.
Martinet predicted that perceptually similar pairs of phonemes with low functional load would merge. This has not been proved empirically; indeed, all empirical tests have come out against it; for example, merged with in Cantonese in word-initial position in the late 20th century although of all the consonants in binary opposition to , only the opposition had a higher functional load than the opposition.