https://www.toppr.com/guides/maths/introduction-to-graphs/delta-symbol/

partly to do with the ancient greeks indeed

pythagoras was one of the early discoverers of algebra, the arabs got to name it though, the lower case of delta, means an angle in euclidian geometry

from teh note about quadratic equations on wikipedia:

Δ is the initial of the Greek word Διακρίνουσα, Diakrínousa, discriminant.

discriminant is the thing that differentiates between two things, the delta of two values is like subtracting one from the other