A chameleon meme was a cloaking device used by the Word Lords' CORDISes, and was a linguistic parallel to a TARDIS' chameleon circuit. CORDISes ran on common words and phrases used throughout the multiverse and disguised themselves as such. According to the Seventh Doctor, these disguises could be jokes, expressions, or any other kind of phrase. (AUDIO: A Death in the Family)
Tardis