A prefix meaning 'previous' (e.g., last year, yesterday). It appears mainly in compounds such as 昨年, 昨夜.
昨年は仕事がとても忙しかった。
Last year was very busy at work.
昨夜は雨が降っていたので、今朝道路が濡れている。
It rained last night, so the roads are wet this morning.
昨今の状況を考えると、対策が必要だ。
Considering recent circumstances, measures are necessary.