In linguistics and especially phonology, functional load (also referred to as phonemic load) refers to the importance of certain features in making distinctions in a language. In other words, a high functional load will make it hard to guess the identity of a phoneme in context when the phoneme has not been heard.
The term "functional load" goes back to the days of the Prague School; references to it can be found in the work of Vilem 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 this 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, given 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 Standard Chinese is as high as that of vowels i.e. 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 e.g. /n/ merged with /l/ in Cantonese in word-initial position in the late 20th century despite the fact that of all consonants in binary opposition to /n/, only the /n/-/m/ opposition had a higher functional load than the /n/-/l/ opposition.
English vowels, for example, have a very high functional load. There are innumerable sets of words distinguished just by their vowels, such as pin, pen, pan, pun, pain, pine. Voicing is similar, as can be seen in pat - bad, sue - zoo. Speakers who do not control these differences make it very difficult for others to understand them.
However, although voicing is generally important in English, the voicing difference between the two fricatives written ⟨th⟩, /θ, ð/, has a very low functional load: it is difficult to find meaningful distinctions dependent solely on this difference. One of the few examples is thigh vs. thy although the two can be distinguished from context alone. Similar is the difference of /dʒ/ (written ⟨j⟩, ⟨ge⟩, etc.) versus /ʒ/ (resulting from /z + j/, or the ⟨j⟩, ⟨ge⟩, etc. in some recent French loanwords), as in virgin vs. version. The difference between the two ⟨ng⟩ sounds, [ŋ, ŋɡ], found in singer and finger, is so unimportant that it makes no practical difference if one mixes them up, and some dialects pronounce the sounds the same in both words. The functional load is nearly zero—-not surprising since the phoneme /ŋ/ originated as a coalescence of [ŋɡ] when word-final.
An ongoing example would be the merger of the AIR and EAR vowels in New Zealand English. The phonetic similarity between words like here and hare does not seem to hamper oral communication in any major way as long as the context is provided. Therefore, those vowels have low functional load in New Zealand English despite their high frequency of occurrences in that dialect.
Another example is the functional load of tone in Mandarin Chinese, which is as nearly as high as that of their vowels. This means that the loss of information when all tones sound alike in Mandarin is approximately equal to that when all vowels sound alike in the language.
- Économie des changements phonétiques: Traité de phonologie diachronique. Par ANDRÉ MARTINET. (Bibliotheca romanica, Series prima: Manualia et cornrnentationes, No. 10.) Pp. 396. Berne: editions A. Francke S. A., 1955
- A manual of phonology. By CHARLES F. HOCKETT(International journal of American linguistics, Vol. 21, No. 4, Part 1 October 1955 = Indiana University publications in anthropology and linguistics, Memoir 11 of IJAL.) Pp. v, 246. Baltimore: Waverly Press (for Indiana University, under the auspices of [the] Linguistic Society of America [and the] American Anthropological Association), 1955.
- Surendran and Niyogi, Quantifying the functional load of phonemic oppositions, distinctive features, and suprasegmentals, chapter in Current trends in the theory of linguistic change. In commemoration of Eugenio Coseriu (1921-2002), Ole Nedergaard Thomsen (editor), Amsterdam & Philadelphia: Benjamins.
- Surendran and Levow, The functional load of tone in Mandarin is as high as that of vowels, Proceedings of Speech Prosody 2004, Nara, Japan, pp. 99-102.