An opposition is an aspect centered on 180°, placing two chart points across the zodiac from each other by longitude. An exact opposition is half a circle; an orb policy allows nearby separations to be grouped with it.
Astrological traditions often use opposition as a symbol of polarity, encounter, or balance between two functions. That reading is interpretive, not a measured force. It does not mean two people or life areas must be enemies.
A reproducible chart states the exact separation and allowed orb. Compare with a conjunction, centered on 0°, and a square, centered on 90°.