Meaning

noun a building where theatrical performances or motion-picture shows can be presented

noun the art of writing and producing plays

noun a region in which active military operations are in progress

originInherited from Middle English theater, theatre, from Old French theatre, from Latin theātrum, from Ancient Greek θέᾱτρον (théātron, “a place for viewing”), from θεᾰ́ομαι (theắomai, “to see", "to watch", "to observe”). Doublet of tiatr.

Etymology from Wiktionary, CC BY-SA 3.0