The term functional load has a long history and first came into use at the turn of the 20th century, often being mentioned in the Prague School. A formal definition, however, was not developed until quite a few years later. The idea behind functional load in a broad sense is that it is the amount of work a given unit (usually a phoneme) does in a language to distinguish words from one another.

André Martinet posited that functional load was a key factor in language change \citep{Martinet1955}; specifically he claimed that phonemes with lower functional load tend to merge, whereas phonemes with higher functional load tend to stay distinct. This hypothesis has been explored to some extent over the years \citep[cf.][]{King1967,Wedel2013a,Wedel2013}.