Refers to the previous night or evening. Slightly more formal and less colloquial than ゆうべ.
See also: 昨夜 (ゆうべ)
I couldn't sleep well last night.
昨夜のパーティーは楽しかった。
The party last night was fun.
I've had a headache since last night.
There was a fire in the neighborhood last night.