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