The Collaborative International Dictionary
Intro-
Intro- \In"tro-\ [L. intro, adv., inwardly, within. See Inter-.] A prefix signifying within, into, in, inward; as, introduce, introreception, introthoracic.
Douglas Harper's Etymology Dictionary
intro-
word-forming element, from Latin intro "on the inside, within, to the inside," from PIE *en-t(e)ro-, suffixed form of preposition *en "in" (see in).