twenty-twenty - Definition from Longman English Dictionary Online

Date:	1800-1900
Origin:	From normal sight being judged on the ability to read letters at a distance of 20 feet

twenty-twenty
noun

twenty-twenty , 20/20 [uncountable]

twenty-twenty vision

the ability to see things normally, without needing glasses:

A pilot must have twenty-twenty vision.

used to say that it is easy to know what you should have done in a situation after it has happened, but you did not know what to do earlier

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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twenty-twenty noun

twenty-two noun