Definition
Ambiguity is the linguistic property by which a single expression can be interpreted in two or more distinct ways. Logicians distinguish two main families. Lexical ambiguity arises from a single word with multiple meanings, such as "bank" denoting either a financial institution or the side of a river. Structural (or syntactic) ambiguity arises from the way a sentence is assembled, so that the same string of words supports different parse trees and different propositions — "visiting relatives can be exhausting" can mean either that one finds the act of visiting tiring or that the relatives themselves are tiring.
Because logic operates on the meanings of statements rather than their surface form, any ambiguous statement is, strictly speaking, not yet a candidate for evaluation. It must first be disambiguated.
Why it matters
How it works
Ambiguity becomes dangerous when an argument moves from one sense of a term to another without the audience noticing. A reasoner who sees the slip can ask which meaning was intended at each step and force the speaker to either pick one or concede the equivocation. Disambiguation typically proceeds by substitution: replace the ambiguous term with a longer phrase that fixes the meaning, and the argument either holds or collapses transparently.
Ambiguity is distinct from vagueness, though the two are often conflated. A vague term such as "tall" has a single meaning whose boundary is fuzzy; an ambiguous term such as "light" has multiple discrete meanings (not heavy, not dark, illumination). Vagueness is a problem of degree; ambiguity is a problem of which meaning. Resolving them requires different moves: vagueness wants a threshold or a context, while ambiguity wants a choice.