Final answer:
A context-free language is ambiguous if there are two or more leftmost derivations for a string.
Step-by-step explanation:
A context-free language is ambiguous if some string has two or more leftmost derivations. A leftmost derivation is a sequence of grammar rule applications that starts from the start symbol and expands the leftmost nonterminal at each step. If a string has multiple leftmost derivations, it means that there are multiple ways to generate that string using the given grammar. This can lead to uncertainty and confusion in understanding the meaning of the string.