noun
deadline; closing date; cut-off
The common modern sense: the final time by which applications, manuscripts, payments, submissions, etc. must be received. Can also refer to the act or point of closing acceptance.
レポートの締め切りは来週の月曜日です。
The deadline for the report is next Monday.
応募の締切
We extended the application deadline by one week.
noun
closing up; keeping closed
Only when written 閉め切り / 閉切り
Refers to the act or state of closing something off, especially doors, windows, or a room. The spelling 閉め切り makes this physical closing sense clearer, though It is a search-only form.
暑い日は部屋の閉め切りを避け、こまめに換気してください。
On hot days, avoid keeping the room closed up and ventilate it frequently.
noun
cofferdam
Civil-engineering term for a temporary watertight enclosure or barrier used to keep water out of a work area. Often written compactly as 締切 in technical contexts.
川の中に締切を設けて、橋脚の工事を行った。
They built a cofferdam in the river and carried out the pier construction.