Yesterday is the day before today.(noun)
If today is Monday, then Sunday is an example of yesterday.
See yesterday in Webster's New World College Dictionary
Origin: ME yistredai < OE geostrandæg < geostran, yesterday (akin to Ger gestern < IE *ĝhyes > Sans hyáḥ, L heri, yesterday) + dæg, day
See yesterday in American Heritage Dictionary 4
Origin: Middle English
Origin: , from Old English geostran dǽg
Origin: : geostran, yesterday; see yester-
Origin: + dǽg, day; see Day.
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