A loanword (from the perspective of historical linguistics at least) is any word that has entered a linguistic system from another linguistic system, as opposed to a native word, which has been naturally passed down through the vertical transmission of that system to new acquirers as far back as we can trace. (Note that there are a lot of edge cases and nuances here, so let me know if you have any specific questions.)

This is a useful categorical distinction as well, since linguistic reconstruction only works on inherited words, and does not work with loanwords or words otherwise created (past the point in history that they entered the linguistic system in question, at least).