Real Dictionary

old maid
countable noun: People sometimes refer to an old or middle-aged woman as an old maid when she has never married and they think that it is unlikely that she ever will marry. This word is likely to cause offence, and should not be used.

old maid in American English
a woman, esp. an older woman, who has never married; spinster
noun: an elderly or confirmed spinster

old maid in British English
noun: a woman regarded as unlikely ever to marry; spinster