Fungible

from Dictionary.com

fungible \FUHN-juh-buhl\, adjective:
1. (Law) Freely exchangeable for or replaceable by another of like nature or kind in the satisfaction of an obligation.
2. Interchangeable.

noun:
Something that is exchangeable or substitutable. Usually used in the plural.

Fungible comes from Medieval Latin fungibilis, from Latin fungi (vice), "to perform (in place of)."

This is one of those words that doesn't mean what it sounds like it should mean. When I first saw the word, I thought it was some kind of food reference, probably unpleasant, like a poisonous mushroom.