The day immediately before today; one day ago.
"I finally finished that report yesterday after staying up late last night."
In plain English: Yesterday is the day that happened before today.
"Yesterday was a busy day for everyone in the office."
Usage: Use yesterday as an adverb to indicate when something happened, but use it only as a noun in the fixed phrase "the other day." Avoid treating it like a regular adjective or modifying nouns directly without the article "other" unless referring specifically to that past date.
On the day before today.
"I finished reading that book yesterday, so I can finally talk about it now."
In plain English: Yesterday is used to say that something happened on the day before today.
"We went to the park yesterday."
The word "yesterday" comes from Old English, originally combining the root for "past" with the word for "day." It entered Middle English and has remained in use to refer to the day before today.