A definition is not merely a name on your map, it's the location in the greater scheme of the map, the longitude and latitude. A definition fixes a notion with respect to some other notions, all of which together form your machinery, your belief network, your map. This machinery may bear no relation to reality, but then, to me, the point of definitions is to be clear, not accurate.

None of which makes it any less true that words-with-definitions are sometimes useful in private thought as well as in communication. For instance, technical terms in mathematics such as "transitive" or "uncountable" can be used robustly in lengthy chains of reasoning largely because they have precise definitions. The fact that when I use such a word (privately or publicly) I have plenty of mental machinery linked with it besides the bare definition doesn't stop it being a definition. (Perhaps you're using "define" in what seems to me to be an eccentric way, such that in fact essentially no words have actual definitions. Feel free, but I don't find that helpful.)