CM is a common way to refer to
television advertising in
Japan.
Yes, I realize that "CM" is not Japanese, and moreover its root, "commercial" is not Japanese, but I call 'em like I see 'em.
When I went to Japan, I learned very quickly there were lots of common words I didn't know; random words that no textbook author would think to teach you, but are used quite frequently. Watching Japanese television, I quickly learned what was meant by "shiiemu no ato!" (When we return!)
"Shiiemu" is as close as Japanese phonetics can get to pronouncing the names of the letters "C" and "M". So there you have it. Now you, too, can talk about the funny ad you saw on the terebi last night. Generic-man informs me that it stands for "Commercial Message."