Star Child is the Reaper King representing the Reapers with a transparent, glowing blue version of the kid that thought Shepard's hand smelled, ran off into an evacuation ship, and got laser'd by Reapers. 

Which makes it a lot more morbid than I first realized. The Reapers are trying to convince you they're doing good/not evil by using the image of a child they brutally murdered/obliterated (along with trillions of other lifeforms).

And yeah, there's a reason I said I'd [i]usually[/i] expect people to only know terms in certain forums. Star Child is a near universal term after the gigantic blow-up at ME3's ending (I can't think of a more major game-specific thing, off the top of my head, other than people complaining/pleading for Half Life 3 to happen already and getting hyped at false omens). I don't even know what else to call 'im.