  We have studied statistical mechanics of a gas of vortices in two dimensions.
We introduce a new observable---a condensate fraction of Onsager vortices---to
quantify the emergence of the vortex condensate. The condensation of Onsager
vortices is most transparently observed in a single vortex species system and
occurs due to a competition between solid body rotation (c.f. vortex lattice)
and potential flow (c.f. multiple quantum vortex state). We propose an
experiment to observe the condensation transition of the vortices in such a
single vortex species system.
