This is a pretty common word but it is so cool. I means more than just a person’s enemy. It is like your worst enemy, your opposite who is always trying to foil your plans. It also makes a cool name for s Star Trek movie!
This word has to be on this site. It can simply mean a dictionary but is can also be a group of words used for something particular or by certain people. Lexicon can also be used to refer to the words a person knows. How big is your lexicon?