epsilon

As nouns the difference between epsilon and eta

is that epsilon is the name for the fifth letter of the Greek alphabet, ε or Ε, preceded by delta (Δ, δ) and followed by zeta (Ζ, ζ) while eta is the seventh letter of the Modern Greek alphabet, the eighth in Old Greek.

As an initialism ETA is

estimated time of arrival.

As nouns the difference between epsilon and upsilon

is that epsilon is the name for the fifth letter of the Greek alphabet, ε or Ε, preceded by delta (Δ, δ) and followed by zeta (Ζ, ζ) while upsilon is the twentieth letter of Classical and Modern Greek; the twenty-second letter of Old and Ancient Greek.