How do I bisect along an axis? Maybe I can't find the answer because I am not familiar yet with all the terminology. I know this is probably a simple answer but I cannot find it. How do I align the plane of a bisect along an axis?

I have an object that I want to bisect where 2 parts meet. The object is aligned to the platform and I want to make a cut through it the Z/Y axes.

Also... there is no tag for the bisect tool
Bisect tool has the Plane Normal option in its settings. It consists of 3 sliders for each axis with values between -1 and 1 which allow to control the orientation of the cutting plane. 

The values below or above 0 will make the custom plane to be oriented towards that axis, while values of 0 or around mean that the plane will be oriented along that axis.

In your case enter 0 value for the Z axis: