1A screw — a metal fastener with helical threads, turned with a screwdriver to fix wood, metal, plastic, or other materials together. The kanji forms (螺子, 捻子, etc.) are essentially unused in modern writing; katakana ネジ is standard in tool and industrial contexts, while hiragana ねじ appears in idioms and softer prose. The idiom ネジが緩む (lit. 'one's screws are loose,' meaning to be slack or unfocused) is in everyday use.
ドライバーでネジを締め直したらガタつきが直った。
When I tightened the screws with a screwdriver, the wobble went away.
眼鏡の小さなネジが取れてレンズが外れた。
A tiny screw came off my glasses and the lens fell out.
家具を組み立てたあと、ネジが何本か余ってしまった。
After assembling the furniture, I had several leftover screws.
最近、彼はちょっとネジが緩んでいるんじゃないか。
Lately he seems a little out of it (lit. his screws are a bit loose).
2A winding key for a clockwork mechanism (watches, music boxes, wind-up toys), or the mainspring itself. Standalone use is somewhat dated now that mechanical watches are uncommon, but the sense lives on in expressions like ねじを巻く (to wind oneself up — buckle down, get serious) and ねじが切れる (to run out of energy, like a wound-down clock).
祖父の古い柱時計は毎週ねじを巻かないと止まってしまう。
My grandfather's old wall clock stops if you don't wind it every week.
そろそろ気持ちのねじを巻き直して、勉強に集中しよう。
Time to wind myself back up and focus on studying.