Standard noun for a period of seven days. Often used with particles like に, で, or の to indicate time.
一週間後に試験があります。
There is an exam in one week.
この仕事は一週間で終わります。
This work will be finished in one week.
Let's meet again in a week.
I go to the gym three times a week.
I finished the report within a week.
I plan to take about a week off.