An original form; primitive word; root.

a simple form inferred as the common basis from which related words in several languages can be derived by linguistic processes

a word ancestor

A lexical form from an earlier stage in the history of a word from which the modern word is derived; root.

Etymological source for a word. The American Heritage Dictionary shows the immediate etymon for words borrowed into English and then more distant etymons in the development of the word. The OED gives considerable etymological information.