Technical definition: Equation in the form y = ax2 = bx + c
Visually: initially increasing/decreasing steeply, with the rate of change becoming shallower until it is flat, then decreasing/increasing (opposite of previous mention) at a slow rate of change with the rate of change becoming steeper7
Equivalent portamento type: ascending or descending slide which has a basically constant rate of change of pitch and then slows as it approaches destination note. N.B. this type of graph is not usually relevant for most portamento curves, as parabolas can drastically vary in shape depending on the variant of equation, which makes them difficult to match to the different portamento curves. Exponential graphs tend to fit portamento curves much more accurately.