hal(o)-

Halogens; salinity or salt.

Greek hals, halo‑, salt or the sea.

The halogens are a group of chemically similar elements that includes fluorine and chlorine, so named because they form salts (called halides) with metals; halogenation is the substitution of hydrogen atoms in methane by halogen atoms, leading to a haloform compound such as chloroform. The sense of salinity appears in halophile (Greek philos, loving), an organism, especially a micro-organism, that grows in or can tolerate saline conditions, and halophyte (Greek phuton, a plant), a plant adapted to growing in saline conditions, for example in a salt marsh.

False friends include halitosis, bad breath, from Latin halitus, breath; although the halibut is a sea fish, its name is actually Middle English for ‘holy flatfish’, because it was often eaten on holy days; and halo comes from Greek halōs, in reference to the disc of the sun or moon.