root word

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A prefix in an English word derived from Greek or Latin.
• noun Alternative form of root. It is the primary lexical unit of a word, which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents. Inflectional stems often derive from roots.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (linguistics) the form of a word after all affixes are removed