Given a grammar, we say that a sequence of strings is a 'W-derivation of V' if 'W' is the first and 'V' the last string in the sequence, and each string of the sequence is derived from the one preceding it by application of one of the rewriting rules [...] Where 'V' is a string of formatives, we say that a 'W'-derivation of 'V' is 'terminated'. We call 'V' a 'terminal string' if there is an '#S#'-derivation of '#V#', where S is the designated 'initial symbol' of the grammar (representing the category "Sentence", and # is the 'boundary symbol' (regarded as a grammatical formative).
- Chomsky (1969), Pag. 66

A terminal string is formed from a preterminal string by insertion of a lexical formative in accordance with the following 'lexical rule': If 'Q' is a complex symbol of a preterminal string and ('D','C') is a lexical entry, where 'C' is not distinct from 'Q', then 'Q' can be replaced by 'D'.
- Chomsky (1969), Pag. 84