Alpha channel; matte
The invisible grayscale layer that says which pixels are solid, which are gone, and which are half there. Transparency as data.
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What it is
Alongside red, green, and blue, a pixel can carry a fourth number: alpha, the opacity. White means solid, black means gone, gray means half there. A matte is that same grayscale information kept as its own image so it can be pulled from elsewhere: a luma matte borrowed from another layer's brightness, a track matte from a shape, a roto matte hand-drawn frame by frame.
Everything in compositing routes through it. Keying produces a matte. Rotoscoping produces a matte. Masking is drawing one. When a comp looks wrong, look at the matte on its own first: view the alpha in isolation and the chewed edges, the holes in the hair, and the gray haze where it should be solid black all become obvious.
Two traps. First, straight vs premultiplied: in premultiplied art the RGB values have already been multiplied by alpha, usually against black. Nothing is baked in as long as the reader knows that. Interpret the file as straight when it was written premultiplied (or the reverse) and the math runs wrong at the edges, which shows up as a dark fringe on semi-transparent pixels, or a light one if it was premultiplied against white, or a colored one if it was premultiplied against a colored matte. Set the interpretation to match how the file was written, don't fix it with a choker. Second, most delivery codecs throw alpha away. H.264 in an MP4 cannot carry it. Export ProRes 4444, QuickTime Animation, or WebM/VP9 with alpha, or ship the fill and the matte side by side and recombine them at runtime.
Ask AI for it
Deliver this animation with a real alpha channel: render the elements over transparency (no background layer, no black fill), keep straight unpremultiplied alpha, and export as ProRes 4444 for editing plus WebM/VP9 with alpha for web. If the target only accepts H.264, instead export a single file with the RGB fill on the left half and the grayscale matte on the right half at the same size, and note the recombination step.