What an aspect is
The degree gap between two planets. That is all.
An aspect is the number of degrees between two planets on the wheel. Ninety degrees is a square, a hundred and twenty is a trine. Nothing mysterious: it is a measured distance.
Why it matters
Why it matters: "Mars in Aries" is a fact. "Mars square Saturn" is a STORY: your drive and your limits are in conflict. Charts speak through aspects, not placements.
0° Conjunction60° Sextile90° Square120° Trine180° Opposition
What does an aspect measure?
The only thing measured is distance; the meaning follows from it.