締 means to tighten, fasten firmly and is a JLPT N1 kanji. It has 15 strokes and uses the Thread radical (糸).
Stroke order · 15 strokes
On'yomi (音読み)
テイ
Kun'yomi (訓読み)
し(まる)、
Radical
糸 · Thread
Strokes
15
How to remember 締
Thread (糸) pulled as taut as an emperor (帝) — tight. 締め切り is a deadline, the eternal companion of content creators; ネクタイを締める means to tighten a necktie.