Hi Burr, it's basically the "other partner" of perpendicular.

For example with this circle with this point on it:



The following is a line perpendicular to the circle at that point:



And this is a line tangent to the circle at that point:




At any point along a curve you can basically get a kind of x/y axis system that is following along the curve like this:





So there are 2 lines above, one is tangent to the curve one is perpendicular to the curve at that point, they cross each other at a 90 degree angle, similar to the 90 degree angle for the standard x/y grid.

- Michael