the 5th letter of the Greek alphabet
"The teacher wrote the epsilon on the board as an example of a lowercase Greek letter used in advanced mathematics."
The name for the fifth letter of the Greek alphabet, ε or Ε, preceded by delta (Δ, δ) and followed by zeta (Ζ, ζ).
"The instructor paused to explain that epsilon is simply the fifth letter of the Greek alphabet, coming right after delta and before zeta."
The word comes from Ancient Greek è psilón, which means "simple E." It was used to describe the letter itself in its unadorned form.