What is it? It's cardio! TM is the new FFitness term for cardio, since "cardio" is a stupid term. The reason exercises that have heretofore been labeled "cardio" are called that is because that's about all they're good for. Eliciting a cardio-respiratory response and nothing more. You can get a CR response, however, from ANY exercise, so to allow those movements to hog the term "cardio" is just not fair. :)
We are replacing this term in our vocabulary with Transitory Movement, since a) movements are better than "exercises", b) all of the previously mentioned "cardio exercises" are actually, when you look at them, common ways for humans to get themselves from one point to another, and c) we don't really like some of the other terms that are in use.
So for your edification, TM, or Transitory Movement, refers to "cardio".