What circuit symbol is this? I know its a diode, but the part number leads nowhere
That is a bidirectional TVS (transient voltage suppressing) diode. Used to clamp voltages, particularly ESD spikes.

Although a bidirectional TVS will protect lines that use unipolar voltages, a unidirectional TVS diode will protect such lines more effectively because they will clamp negative spikes closer to GND via forward conduction rather than waiting for the negative spike to get large enough to cause the TVS to break down in reverse.

You can also make a bidirectional TVS diode by placing two unidirectional TVS diodes in series facing opposite directions. That's all a birdirectional TVS diode is, except it's just in a single package instead of two. Don't forget to account for the forward voltage drop of the one diode by subtracting it from the reverse breakdown voltage of the other.