(slang) A hydrogen bomb.

(figuratively) The hour at which any major event planned for the future is set to begin, as used in United States military.

(euphemistic) The word hell/Hell. / (humorous) Any word beginning with h that is not normally taboo but is considered to be so in the given context, such as horny.

(fandom slang) The ship of characters Harry Potter and Hermione Granger of the Harry Potter series.

(Judaism) b'ezrat hashem (Transliterated form of ב״ה, written at the top of documents). / (Judaism) baruch Hashem

(mining) The part of a plunger pump that contains the valve.

(astronomy) Fe/H (metallicity)

A shackle shaped like an H, or two Us joined together to give a waisted appearance, instead of the more usual single U shackle, or D-shackle.

(initialism) hematoxylin and eosin: a histological stain widely used in medical diagnosis

plural of H-shackle