I keep thinking monogram, but that's not it.
Well, it may technically be a monogram, but that's not the word I'm looking for, I found it in a book of "INSERT WORD HERE"
It's driving me nuts.
EDIT: To help you help ME, it was from a book i found at the library, years ago. It had all sorts of letter combinations, mostly 2-letter combos, but sometimes 3-letters. If that helps...somehow...
EDIT: Got it. The word I was looking for was "cypher".