observer, bystander, onlooker, spectator, eyewitness, viewer, beholder, watcher, testifier, deponent, attester, perceiver, looker-on, witnesser, seer
confine - imprison - incarcerate - hold - restrain - arrest - intern - lock up - take into custody - keep - apprehend - capture - seize - nab - detain - remand - impound - quarantine - withhold - constrain - restrict - check - impede - hinder - obstruct - delay - stall - stop - prevent - bar - block - curb - hamper - inhibit - interrupt - retard - slow - stay - suspend - tether - trammel
To detain someone means to keep them in official custody, typically for questioning about a crime or in politically sensitive situations. This action is usually carried out by law enforcement officers to prevent the person from fleeing or committing further offenses while an investigation or legal procedure is ongoing.