Aspect
The angular distance between two points on the ecliptic, counted only when it lands on one of a few specific values.
The common five: conjunction (0°), sextile (60°), square (90°), trine (120°), opposition (180°). All of them divide the circle evenly — into one, two, three, four or six parts.
Aspects don't need to be exact; the allowance is called the orb. So "a square" really means "90° give or take a few degrees".
Squares and oppositions get labelled hard, trines and sextiles easy. The labels wear thin with use — an aspect is a geometric relationship between two forces, and tension isn't necessarily bad or ease necessarily productive.