eta; Η, η

(Greek: the seventh letter of the Greek alphabet; Η, η)

1. The seventh letter of the Greek alphabet (H, η), which is represented in the English alphabet as "e" (AY tuh) or "ē" (EE tuh).
2. As a numeral, it refers to 8.
3. A symbol for the viscosity of fluids.
4. In chemistry, it denotes the position seven atoms from the carboxyl group or other primary functional group (functional group characteristic of all organic acids).

Eta is for helios, the divine personification of the "sun", and for all of the helio- words in English; such as, heliocentric, heliotrope, heliozoan, helium, etc. that are derived from Greek helios, "the sun".

Eta is in fact a double for Helios and its offspring, since this letter underlies the Greek orthographic signs for both H and e (long e, or ee).

—These last two paragraphs are based on excerpts compiled from
Alpha to Omega, The Life & Times of the Greek Alphabet
by Alexander & Nicholas Humez; David R. Godine, Publisher;
Boston, Massachusetts; 1985; page 45.
Eta Aquarids
In astronomy, a meteor shower derived from Halley's Comet that reaches peak activity around May 4.

It appears to originate near the star Eta in Aquarius.

Eta Carinae Nebula
In astronomy, a large region of ionized hydrogen surrounding the erratically variable supergiant star Eta Carinae.

You can see all of the letters of the Greek Alphabet on this page.