In linguistics, functional load, or phonemic load, is the extent to which a particular phoneme helps distinguish words from one another in a language. A phoneme is the smallest portion of sound that helps convey meaning. The same phoneme can have a functional load that is high in one language, but low in another.
One way to understand this concept is by looking at minimal pairs — that is, two words that are the same except for one phoneme. For example, "rub" and "rut" form a minimal pair because they are pronounced the same except for the final sound. Changing the /b/ sound to a /t/ sound changes the meaning of the word. In English, the /b/ to /t/ change will always produce either a different word or a nonsense word. That means that /b/ and /t/ are phonemes with a very high functional load.
Other phonemes are not as important in distinguishing words from one another. The short /i/ and short /e/ sound sometimes, but not always, distinguish two words from one another. For instance, "pin" and "pen" are a minimal pair. On the other hand, the word "friend" can be pronounced with either an /i/ or an /e/ sound with no difference or confusion in meaning. These sounds are even interchangeable in some English dialects, and so have a fairly low functional load.
Similarly, which syllable is stressed sometimes changes the meaning of an English word. "Per fect" and "per fect" are a minimal pair because they are the same except for their stress. The word "aluminum," however, can be pronounced either "a lum in um" or "a lu min um." There are relatively few minimal pairs in English that are determined by their stress, so stress is a phoneme with a fairly low functional load.
Pitch is another phoneme that may have differing amounts of meaning attached to it in various languages. In Mandarin Chinese, changing the pitch or tone of a word or syllable may produce an entirely different, unrelated word. This is never the case in English though. Therefore, pitch has a high functional load in Mandarin Chinese, but none at all in English.
Some languages also make a distinction between an aspirated /ph/ and an unaspirated /p/. Changing from one to the other may form a different word, so it has a functional load in languages like Hindi. Many English speakers, however, cannot even hear the distinction between these two sounds. Since there are no English minimal pairs that depend on the difference between those sounds, it has no functional load.