The Trace [[Velocity]] of a [[Wave]], denoted $v_{\text{tr}}$, is a property where if the wave is hitting a surface and some angle with a [[Wave Speed]] $c$, the [[Trace Velocity]] is how fast the point of contact between the wave and the new medium along the boundry between that medium. If the angle is at $90\degree$ to the medium, then the trace velocity is effectively infinity, while if the angle is [[Parallel]], the trace velocity is $0$.