Chomsky Normal Form

(computing theory) A context-free grammar in which the right hand side of any production rule consists of either one terminal symbol or two non-terminal symbols.

(computing theory) A context-free grammar in which the right hand side of any production rule consists of either one terminal symbol or two non-terminal symbols (neither of which is the start symbol) or the epsilon (i.e. null string) symbol, but only if the left hand side is the start symbol. (Note: this is the full-fledged version, whereas the previous definition may be said to be defining Chomsky reduced form.)