According to the scheme set out by Leonard Bloomfield in his book Language (1933), the tagmeme is the smallest meaningful unit of grammatical form.
Bloomfield's term was adopted by Kenneth Pike and others to denote what they had previously been calling the grammeme (earlier grameme).
Tagmemics as a linguistic methodology was developed by Pike in his book Language in Relation to a Unified Theory of the Structure of Human Behavior, 3 vol.
It was primarily designed to assist linguists to efficiently extract coherent descriptions out of corpora of fieldwork data.
Tagmemics makes the kind of distinction made between phone and phoneme in phonology and phonetics at higher levels of linguistic analysis (grammatical and semantic); for instance, contextually conditioned synonyms are considered different instances of a single tagmeme, as sounds which are (in a given language) contextually conditioned are allophones of a single phoneme.