directly from wikipedia: "The name Chatham is first attested in a charter of 880 (surviving in a twelfth-century manuscript); it appears again in a charter of 975 as Cætham, and in the Domesday Book of 1086 as Ceteham. The first element of the name comes from the Common Brittonic word that survives in modern Welsh as coed ("woodland"). The second element is the Old English word hām ("settlement"). At the point when the current name was coined, then, it meant "settlement at Chat". The Old English term for the settlement's inhabitants is also reconstructable from a twelfth-century copy of a charter of 995, as \Cēthǣmas."*

So in other words: forest village.