detain

To detain somebody is to hold them back, or to keep them in custody. Two senses, one polite and one very serious.

The polite one is formal English for delaying somebody, as in I won't detain you long, which is what a speaker says before going on for another ten minutes. The legal sense is far weightier, since being detained by police means being held, and detention without charge is a live political argument in most countries. Schools have detention as a punishment, from the same root. The noun is detention or detainment.