Definitions of yesterday:

• noun:   the day immediately before today

  Example: "It was in yesterday's newspapers"

• noun:   the recent past

  Example: "Yesterday's solutions are not good enough"

• adverb:   on the day preceding today

  Example: "Yesterday the weather was beautiful"

• adverb:   in the recent past; only a short time ago

  Example: "I was not born yesterday!"