Real Dictionary

Mesolithic in American English
designating or of an Old World cultural period (c. 10,000-c. 8000 b.c.) between the Paleolithic and the Neolithic, characterized by the earliest exploitation of local and relatively permanent food resources and the use of microliths
adjective: of, pertaining to, or characteristic of a transitional period of the Stone Age intermediate between the Paleolithic and the Neolithic periods, characterized by adaptation to a hunting, collecting, and fishing economy based on the use of forest, lakeside,

Mesolithic in British English
adjective: of or relating to the Mesolithic
noun: the period between the Palaeolithic and the Neolithic, in Europe from about 12 000 to 3000 bc, characterized by the appearance of microliths