The Rebbe Rashab founded Yeshivas Tomchei Tmimim, whose purpose was that in it should be learned “G-d’s Torah, the exoteric and the esoteric, as a whole.” That is why the Yeshivah was called “Tomchei Tmimim,” and its students called Tmimim (Tmimim — plural of “tomim” meaning whole) — for there the synthesis of the exoteric and esoteric was manifest. Although the exoteric and esoteric were learned before the founding of Tomchei Tmimim, they were not learned together in the same building and day; nor was the esoteric given the same importance as the study of the exoteric. Thus the contribution of the Rebbe Rashab was the synthesis of the exoteric (earth) with the esoteric (heaven).
Today, the birthday of the Rebbe Rashab gives strength for all students (past and present) of Tomchei Tmimim to increase in all things associated with the spirit of Tomchei Tmimim, and to act accordingly. Study in Tomchei Tmimim is not something in the past; even one moment’s study in it is an eternal matter, and such a person is always a Tomim. The only choice in this matter is whether to reveal this eternal bond by acting in the spirit of Tomchei Tmimim, or the reverse G-d forbid.
http://www.sichosinenglish.org/books/sichos-in-english/15/25.htm Emphasis mine.
Practically, the title is used on anyone who learned in a Lubavitch affiliated yeshiva. As a title, it's used mainly by Aliyas (as in Yaamod, HaRov [or HaBachur] HaTomim Ploni ben bloni Maftir).
It's also used as an adjective (like "He's a Tomim"). However, (in my experience) it's mostly used to describe where he learned ("He was a Tomim from Otvotzk/Lubavitch)