|Definition - A language with a high morpheme-per-word ratio. Some languages of this type are Sanskrit, Spanish, French, Russian, Polish, Navajo, Nahuatl, Mohawk, and Quechua.|
Oxford English Dictionary - The term's first citation is from 1816:
"The third class [of languages] would … be that in which the principal parts of speech are formed by a synthetical operation of the mind, and in which several ideas are frequently expressed by one word. Such are what are called the Oriental languages, with the Latin, Greek, Slavonic, and others of the same description. These I would call synthetic."
(P. S. Duponceau in Trans. Hist. & Lit. Comm. Amer. Philos. Soc. (1819) I. 401)