The technical term for a single base unit of independent meaning such as a word, especially a word in its 'base form' as found as a head word in a dictionary's pages shown in bold, e.g. interest, bridge, mouse but also including some 'phrasal verbs' that have separate meanings from their constituent lexemes, e.g. 'to see to', 'to break down', 'to put up with', 'to wind up'.
The collection of lexemes that forms your vocabulary is called your personal lexicon. A dictionary is another kind of lexicon.