"a guard," is used of the place where persons are kept under guard (akin to phulax, "a keeper"), and translated "ward" in Act 12:10. See CAGE, HOLD (Noun), IMPRISONMENT, PRISON, WATCH.
<2,,5084, teresis>
primarily denotes "a watching" (tereo, "to watch"); hence "imprisonment, ward," Act 4:3 (AV, "hold"); Act 5:18, RV, "(public) ward" [AV, "(common) prison"]. See HOLD (Noun), KEEPING, B, PRISON.
Note: For "were kept in ward," Gal 3:23, see GUARD, B, No. 3, KEEP, No. 6.