tense 2 BrE AmE ( also tense up ) verb [intransitive and transitive]
[ Word Family: noun : ↑ tenseness , ↑ tension ; verb : ↑ tense ; adverb : ↑ tensely ; adjective : ↑ tense ]
to make your muscles tight and stiff, or to become tight and stiff:
Relax, and try not to tense up so much.
Every time the phone rang, she tensed.