Last week (the week before the current one). Often used similarly to 先週, though 昨週 can appear more in written language.
昨週は仕事が忙しくて、休めなかった。
I was busy with work last week and couldn't rest.
昨週の会議で新しい計画が発表された。
A new plan was announced at last week's meeting.
友達に昨週会ったばかりだ。
I just saw my friend last week.