Aspect
An aspect is a specific angle between two planets, and the five classical ones are the conjunction, sextile, square, trine and opposition.
The angles are divisions of the circle: 0, 60, 90, 120 and 180 degrees. Two planets in aspect are said to see each other, and the angle describes how, from the ease of the trine to the friction of the square.
An aspect counts when it falls within an orb, the tolerance around the exact angle, and it is applying when the faster planet is still approaching exactness.
Frequently asked
Are squares bad and trines good?
That is the shorthand, not the tradition: squares describe tension that produces movement, trines ease that can go unused.
What are minor aspects?
Divisions such as the quincunx and semi-sextile, used with tighter orbs and read as secondary.