Delta (Δ) is widely used to denote differences.
Pi (Π) is a mathematical constant everyone should know (ok, that's lowercase π but that looks almost the same as uppercase Π, depending on the font)
Omega (Ω) is the symbol for Ohm, the SI unit for electrical resistance. Christians may also be familiar with the letter for
other reasons.
Chi (Χ) is quite well-known from the statistical
Chi-squared test. It looks a lot like an X, so if you don't know it, it's not hard to describe it.
Lambda (λ) is known among computer scientists for lambda calculus / functional programming. Among gamers it's probably best known as the
Half-Life logo.
Gamma (γ) is maybe not that widely known. It's the third letter of the Greek alphabet though, so people may have heard "alpha beta gamma" at some point (e.g. radiation).