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In a TV show: "And now a word from our sponsor. Stay tune..."
In this case, a word means a "CM"? Is this usage rare?
" a word" here means that the announcer is telling you that a commercial message (I assume this is what you mean by "CM") break is next on the program....it is never just a word though!
This is a very commonly used phrase in the radio and television media.
meaning of the "word" 
Linear Mode
