Last night; the evening or night of the previous day (often spoken as 'yunbe' in informal speech).
昨夜はよく眠れなかった。
I couldn't sleep well last night.
昨夜の会議で重要な決定が出た。
An important decision was made at last night's meeting.
昨夜、雨が降っていたらしい。
It seems it rained last night.
昨夜あなたに電話したけど、出なかった。
I called you last night, but you didn't pick up.