How many letters does the top answer for “a system of words used to name things in a particular discipline” have?

The most common answer is nomenclature (12 letters).

Crossword clue: a system of words used to name things in a particular discipline

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

nomenclature 12 letters wn-def

a system of words used to name things in a particular discipline

terminology 11 letters wn-def

a system of words used to name things in a particular discipline