Boyd’s Bible Dictionary:
(other speech). That figure of speech by which a subject is set forth under the guise of some other subject (Gal. 4:24).
Concise Bible Dictionary:
The word ἀλληγορέω, occurs only in Galatians 4:24. The passage does not mean that Abraham having two sons was an allegory: it was history, but that history had an allegorical application, which Paul, by the Holy Ghost, fully explains. The Greek word signifies “to speak otherwise,” and an allegory is a description of one thing under the image of another.