According to Dr. Bill's source (http://humanities.byu.edu/rhetoric/Figures/TMESIS.HTM) tmesis is interjecting a word or phrase between parts of a compound word...

According to tsuwm's source (http://www.dictionary.com/wordoftheday/archive/1999/05/26.html) it's the act of splitting the word or words apart.

So which is it?

Don't make me use my OED