A tholos (from Ancient Greek θόλος (thólos) 'conical roof, dome'; pl. tholoi; Latin: tholus; pl. tholi) is a form of building that was widely used in the Greco-Roman world. It is a round structure with a circular wall and a roof, usually built upon a couple of steps (a podium), and often with a ring of columns supporting a conical or domed roof.
It differs from a monopteros (Ancient Greek:ὁ μονόπτερος from the Polytonic: μόνος, only, single, alone, and τὸ πτερόν, wing), a circular colonnade supporting a roof but without any walls, which therefore does not have a cella (room inside). Both these types are sometimes called rotundas.