tmesis (t(uh)-MEE-sis) - n., (Rhetoric) the interpolation of one or more words between syllables of another word.
infix (IN-fiks) - n., (Linguisitics) a morpheme inserted inside an existing word.
The difference here is that tmesis is the term used when rhetoricians see it happen and infix when linguists see it. This usually, but not always, happen between parts of a compound, as in-freakin'-credible as that may seem -- an example that does break apart parts of a compound is "what man soever." And that's the abso-bloody-lutely truth.
Tmesis was borrowed around 1580 from Latin, from Greek tmēsis, act of cutting, from temnein, to cut, making it a cognate of suffix of appendectomy and other surgical removals. Infix is a back-formation from Middle English infixed, stuck in, from Latin īnfīxus, past participle of īnfīgere, to fasten in, from in-, in + fīgere, to fasten.