"Ejectile" is a shorthand combination of projectile and eject commonly used in physics, especially physics where something's hit by something else and yet another thing (otherwise it'd be simpler to keep calling it the projectile) goes flying off. (or I suppose it could be used anyplace something's ejected ballistically. So much for completely avoiding Freud.)
Ok, so that didn't make much sense.
To demonstrate the concept a little more competently, a concrete example is in order.
If you can call nuclear particles concrete... after all, the liquid drop model works fairly well.

Let's let A(p,a)B be a nuclear reaction. In order: A's a nucleus. It's just innocently sitting there, not unlike a deer in headlights, when a beam of p - which we'll suppose are protons just to have something that actually begins with "p" - comes flying in. Some of the protons get absorbed into the nucleus and knock off "a" particles - let's call them alphas. So the alpha particles go flying off, leaving behind a somewhat modified nucleus which can no longer properly be called A -- it's a different element, since the number of protons has changed. So let's call it B.

Ok, now take a step back. What was the ejectile here? The alphas. It couldn't have been anything else. A and B didn't move - at least no more than recoil made them, and they're probably big compared to the protons or alphas. (aka recoil doesn't do much. like Earth doesn't move much when you jump up and down.) And the protons? They were incoming, not ejected - so they were the projectiles! Et voila - process of elimination in action. Any process on any scale that is analogous to this one can be labeled similarly.

While I understand that this is probably way over-explained for most of you, it's a nice change from the level of the lecture I sat through this morning. Indulge me!

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