Equivalence is the dictionary-approved form of the noun meaning the state or condition of being equal or interchangeable. Equivalency has a few recognized uses, but mostly it’s an unnecessary variant of equivalence. Whenever you’re in doubt, use equivalence.
Examples
Equivalence is preferred to equivalency in almost all contexts—for example:
There is no moral equivalence between Israel’s defensive measures and the Syrian dictatorship’s brutal repression of its own people. [Washington Post]
There is no equivalence between Obama and George W Bush. [Guardian]
His argument is more nuanced than drawing a moral equivalence between moderates and fundamentalists but it is a dangerous path to tread down. [Sydney Morning Herald]
The main use of equivalency is in high-school equivalency, college equivalency, and related phrases, which mostly appear in North American publications—for example:
… but jailed students historically had just gotten equivalency degrees. [New York Daily News]
He got his high school equivalency, landed immigrant status, then Canadian citizenship. [Globe and Mail]
Outside these education-related uses, equivalence is usually the safer choice.
Ngram
This Ngram, which charts the use of equivalence and equivalency in English books published from 1800 to 2000, shows equivalency winning in a landslide.
