What is the meaning of Η?

The metric tensor in quantum field theory.

The coefficient of viscosity.

Efficiency in engineering.

Sommerfeld parameter.

Learning rate.

a velar nasal (IPA ŋ).

Lower-case eta, the seventh letter of the ancient Greek alphabet. Its name was ἦτα and it represented a long open-mid front unrounded vowel: /ɛː/. It is often transliterated as either e or i. It is preceded by ζ and followed by θ.

Lower-case eta, the seventh letter of the modern Greek alphabet. Its name is ήτα and it represents a close front unrounded vowel: /i/. It is often transliterated as either e or i. It is preceded by ζ and followed by θ.

nominative feminine singular of ο (o) (the)

Source: wiktionary.org