A loanword is a word (or more precisely, lexeme) that at some point in the history of a language was borrowed from another language. A word that is not a loanword is called a native word (or inherited word).
The spelling loan word is also sometimes used.
Apparently loanword was created on the model of German Lehnwort. In English, the verb to loan is hardly used in a linguistic sense, but in German entlehnen 'to loan' is the normal linguists' word for 'to borrow'.