Why we name one side as the perpendicular of an angle but does not actually define it?

Question

If I have a right angled triange:
$\qquad \qquad \qquad \qquad$
I was wondering why we name the sides like this? The base of $A$ kind of make sense. But the perpendicular of $A$ what relation does it have with the angle $A$? It doesn't even define the angle $A$. Why are we using this terminology? And not use e.g. perpendicular of base of $A$? Or this is what we mean? Because it seems confusing to me.