An invincible anti-air move is an anti-air move with an invincible hitbox at its outset. This is useful as its own term because each one of its component properties can work very well together; for example, the “Ume-Shōryū” is a technique made famous by Umehara Daigo playing Street Fighter II, in which he would use the invincibility at the outset of a Shōryūken to go through, say, Dhalsim’s far-reaching pokes as if he were psychic (and if he had been reasonable and jumped, he would have basically been saying HIT ME, SIM).
This does not include moves for which the invincibility is only while rising or rushing. The usefulness of an invincible anti-air move lies in its ability to avoid getting hit simply by activating it on time.