Subtractive morphology

Definition
Subtractive morphology is a term which refers to the situation where the semantically more complex category is represented by a phonologically simpler form.

Example
The genitive plural of feminine and neuter nouns in -a/-o in Russian (kniga 'book', mesto 'place') are formed without any (overt) affixation (knig, mest).

Links

 * Utrecht Lexicon of Linguistics