tight‧en also tighten up
to close or fasten something firmly by turning it [≠ loosen]:
Tighten the screws firmly.
I'd put the new tyre on, but I hadn't tightened up the wheel.
2 [intransitive and transitive]
if you tighten a rope, wire etc, or if it tightens, it is stretched or pulled so that it becomes tight:
When you tighten guitar strings, the note gets higher.
The rope tightened around his body.
3HBH [intransitive and transitive]
to become stiff or make a part of your body become stiff [≠ relax]:
His mouth tightened into a thin, angry line.
Tighten up the muscles of both arms.
to control a place or situation more strictly:
Rebel forces have tightened their hold on the capital.
to hold someone or something more firmly:
Sarah tightened her grip on my arm.
to make a rule, law, or system more strict [≠ relax]:
Efforts to tighten the rules have failed.
tighten up on something
a range of measures to tighten up on illegal share dealing
to try to spend less money than you used to:
Businesses were tightening their belts and cutting jobs.
to try to force someone to do something, by threatening them or making things difficult for them - used in news reports:
Closing the border would tighten the screws on the terrorists.
8 [intransitive] American English
if a race or competition tightens, the distance between the competitors becomes smaller:
He expects the presidential race to tighten.
tighten upphrasal verb
tighten something ↔ up
We have tightened up the defence and are winning matches as a result.