/ep"seuh lon', -leuhn/ or, esp. Brit., /ep suy"leuhn/ , n.
1. the fifth letter of the Greek alphabet (E).
2. the consonant sound represented by this letter.
3. Math. an arbitrarily small quantity, used to indicate that a given quantity is small, or close to zero.
[ è psilón bare, simple e (as opposed to diphthongal spellings which in later Gk represented the same sound) ]