Animation curve
A function from time to value — usually keyframes joined by tangent-controlled interpolation segments — that drives any animatable property.
In the curve editor (the graph editor) each keyframe is a point with tangents controlling its incoming and outgoing slope: linear, flat (hold the value), or Bézier handles for smooth ease-in and ease-out. The curve is therefore both the keys and the shape of the motion between them.
Any float property can be curve-driven: position, rotation, a material value, camera field of view, a light's intensity. Engines like Unity expose the curve as its own reusable asset you wire in anywhere, not just onto skeletons. It is the concrete form of tweening and easing — the shape of the curve is the feel of the movement.