The word "theater" has multiple meanings, referring to both a performing art form and a physical space: 

Performing art

Theater is a collaborative art form that uses a combination of voice, movement, words, and visual elements to convey meaning. It can include live performances, scripted works, and improvised performances. Theater can also include dramatic forms like film, television, and other electronic media. 

Physical space

Theater can also refer to a building or space where performances take place. Theaters typically have a stage where the performance takes place. The design of theaters has evolved over time to meet the audience's needs for seeing and hearing the performers. Different types of theaters have different architectural designs to suit the types of performances they host. For example, a "thrust" stage has the stage extend out into the audience, which surrounds the stage on three sides. This design is meant to create a sense of intimacy between the audience and the actors. 

Theater of war

A theater of war is the area or region where a war or conflict is taking place. 

The word "theater" comes from the Greek word theatron, which means "a place of seeing".