A small city-state in NW Boiotia (central Greece, 25 km. east of Delphi, Barrington Atlas map 55 grid D4), near the border with Phokis (Phocis) and commanding the route down the valley of the R. Kephisos (Cephisus); supposedly named after an eponymous Chairon (chi 178; see also chi 177). See generally M.H. Hansen, "An inventory of Boiotian poleis in the archaic and classical periods", in M.H. Hansen (ed.), Introduction to an Inventory of Poleis (Acts of the Copenhagen Polis Centre 3: Copenhagen 1996) 73-116, at 81-2.
Its principal claim to fame was in providing an area of level ground suitable for large pitched battles: J.F. Lazenby in OCD(4) p.303 ("Chaeronea, battles of") mentions the best-known two, in 338 and 86 BCE, but at least one other is known (245 BCE: Polybius 20.4.4-5; Plutarch, Aratus 16.1).
[1] Only the second of these forms of the ethnikon is otherwise attested (Stephanus of Byzantium s.v., Polybius 27.1.4, SEG 15.282, etc.).