Theatre can refer to a performing art or a building where performances take place:

Performing art

A collaborative art form that uses live performers, voice, movement, and visual elements to present a real or imagined event to an audience. Theatre includes live, scripted, and improvised work, as well as dramatic forms like film and television. 

Building

A structure where theatrical works, musical concerts, and performing arts are presented. The building defines the audience and performance spaces. The word "theatre" comes from the Greek word theatron, which means "a place of seeing". The design of theatres has evolved over time to meet the needs of the audience to see and hear the performers.