Yesterday (the day before today). The colloquial reading is きのう.
昨日は友達と映画を見た。
I watched a movie with a friend yesterday.
昨日の会議は長引いた。
Yesterday's meeting ran long.
昨日まで雨が続いていた。
It had been raining until yesterday.
昨日、駅で財布を落とした。
I dropped my wallet at the station yesterday.