Theatre-in-the-round is a way of staging a play where the audience sits all the way around the acting area, in a circle or a square, instead of all sitting on one side looking at a picture-frame stage. The actors are in the middle, with people on every side of them.

This changes a lot for everyone. There is no big painted background, because a wall on one side would block half the audience, so the set is usually just furniture and props. The actors have to keep turning and moving so that no group of watchers stares at their back for too long. And the front row is very close, sometimes close enough to touch the actors, which makes the whole thing feel more intimate and a little more risky than a normal show.

The word is used both for the method and for the building: you can say a play was performed in the round, or that a city has a theatre-in-the-round. In fact the short phrase 'in the round' is what people say most often — 'they staged Hamlet in the round'. British English writes theatre; American English writes theater-in-the-round. Note the hyphens, which hold the whole phrase together as one noun.