Monopteros

A monopteros (Ancient Greek: ὁ μονόπτερος, from: μόνος, 'only, single, alone', and τὸ πτερόν, 'wing'), also called a monopteron or cyclostyle, is a circular colonnade supporting a roof but without any walls.

[1] Unlike a tholos (in its wider sense as a circular building), it does not have walls making a cella or room inside.

An example of this is the Monument of Lysicrates in Athens, albeit with the spaces between the columns being walled in, even in ancient times.

The Temple of Roma and Augustus on the Athenian Acropolis is a monopteros from Roman times, with open spaces between the columns.

These also have only a decorative function, because they are not needed in order to provide an entrance to a temple that is open on all sides.