A hum is a sound with a particular timbre (or sound quality), usually a monotone or with slightly varying tones, often produced by machinery in operation or by insects in flight. To hum is to produce such a noise, with humans vocally producing such a sound, humming, though most often with a melody.

The action of humming is produced simply by producing a wordless tone, such as in scat singing or vocables (wordless forms of singing), but with the mouth completely closed so that the sound emerges from the nose. This necessarily keeps the volume at a low level, so humming is often not useful in musical productions, but is nonetheless sometimes used.

Often, people hum either individually or when in group singing when they don't know the words or lyrics.

Frank Zappa used hum as a pun in his song, Dynamo Hum, with the reference being both to the hum made by a generator (dynamo) in operation, and to the humming action of a woman giving fellatio.

Hum, like Zahumlje, is a historic name for areas of today's Herzegovina.

Hum, locality in central Istria, near Roč in Istria county (Croatia), considered to be the smallest town in the world with just 22 residents.

Hum is one of the transliterations of the sixth syllable in Buddhist "Six syllable mantra" Om mani padme hum.