Give Me a Synonym for "Dictionary"

Earlier today, in an attempt to relax for half an hour, in between my studies, I played a silly game that sometimes makes me more nervous than calm. This was the case today, but this time the players had me exclaim "how stupid can one be?!!". Instead of enjoying the game, I had to fight with people, while trying to explain them the Thesaurus IS a dictionary. "No, dictionaries give definitions of words, this one gives words with the same meaning". Am I going crazy or this is plain ignorance? "OK", I said, "it's a SYNONYM dictionary"!
I don't know, I was just extremely shocked by this episode... blah!