Keyframe
A marker pinning a property to a value at a specific time. Set two and the software invents every frame in between.
See it
What it is
You do not animate frames, you animate values. A keyframe says 'opacity is 0 at second 0 and 100 at second 0.5'. Everything between is interpolation, invented by the software. The word comes from cel animation, where the lead animator drew the key poses and assistants drew the in-betweens. Same division of labor, different assistant.
Use keyframes for anything time-specific: a title sliding in, a mask following a hand, a volume dip under narration. The shape of the interpolation matters far more than the keyframes themselves. Linear keyframes read as robotic, because nothing in the physical world starts and stops at full speed. Two keyframes with eased handles beat eight with flat ones.
Gotcha: over-keyframing. Long strings of keyframes fighting each other produce jitter and are miserable to retime, since moving one shifts the rhythm of all of them. Spatial keyframes have a second trap: they carry both a value curve and a motion path, so a move can be perfectly smooth in time while its path still turns an ugly corner. Fix the path in the viewer, the timing in the graph editor.
Ask AI for it
Animate this logo with keyframes on scale and opacity: frame 0 at 80 percent scale and 0 opacity, frame 12 at 103 percent scale and 100 opacity, frame 18 settling to 100 percent scale, then hold to frame 90. Ease out of the entrance keyframes with Bezier handles at roughly 70 percent influence, and use no linear interpolation anywhere. Add motion blur at a 180 degree shutter so the fast part of the move smears. Show me the graph editor curves alongside the result.