H²

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1. Physics. Hamiltonian
2. Physics. henry
3. high
4. Baseball. hit
5. hot
6. humidity

The noun has 4 meanings:

Meaning #1: a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe
  Synonyms: hydrogen, atomic number 1

Meaning #2: a unit of inductance in which an induced electromotive force of one volt is produced when the current is varied at the rate of one ampere per second
  Synonym: henry

Meaning #3: the 8th letter of the Roman alphabet

Meaning #4: a narcotic that is considered a hard drug; a highly addictive morphine derivative
  Synonyms: heroin, diacetylmorphine, horse, junk, scag, shit, smack