Lexical Gap

Definition:

A lexical or accidental gap is a well formed but absent word in a language.

Actual words are a subset of possible words, and the words that are in PossibleWords but not in ActualWords are the lexical gaps.

Examples:

"Communist" is a valid English word, and its existence means that "Communite" and "Communian" were blocked, becoming lexical gaps.

Other References:

Wikipedia