Epsilon/ˈepsɪlɑːn/ Definition 1 of 4Pro Tip 1/2Greek Origin Remember, 'epsilon' is from the Greek alphabet, so it's not used in regular English spelling. The Greek word for 'epic' starts with an epsilon. Pro Tip 2/2Not a Number Don't confuse 'epsilon' with the term 'e', which refers to a mathematical constant. Unlike 'e', epsilon simply represents the letter 'E' in Greek.