make that pi/2, but yeah, the tangent function is related with this tangent line, that touches the unitary circle centered at (0,0) at a single point of the positive half of the x axis. but I think this tangent line is not the one that originated the tangent function. the tangent line at point A, in https://en.wikipedia.org/wiki/File:Circle-trig6.svg, intersects the x axis at E (unless that tangent line is parallel to the x axis), and the length AE is probably the original definition of the tangent function.

https://en.wikipedia.org/wiki/Tangent_(trigonometry)#Unit-circle_definitions is where that drawing appears, and that page has a bit of a bug under the linked section: it defines various points, A, B, C, D and E in the text, but only some of them correspond to the named points in the figure, which makes it unnecessarily confusing to read. if someone would be so kind as to edit the talk page to point that out (Wikipedia won't allow me to do so while using Tor), that would IMHO be a useful contribution to Wikipedia