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).