Bézier curve
A smooth curve steered by control points that pull on it without touching it.
See it in SVG path animator. Pick the Wave preset and watch its curves draw themselves.
In plain words
A Bézier curve starts at one point and ends at another. Between them sit control points, which pull the curve toward them like magnets. The curve never touches the control points, but moving them reshapes it smoothly. Fonts, SVG shapes and drawing apps are built from these curves. Easing curves in animation are Bézier curves too.