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A hum is a sound made by singing a wordless tone with the mouth completely closed, forcing the sound to emerge from the nose. To hum is to produce such a sound, most often with a melody. It keeps the volume at a low level, so humming is rarely used in musical productions, with some exceptions such as scat singing and vocables. Humming is sometimes used to keep the melody when the singer does not know the lyrics.

A hum has a particular timbre (or sound quality), usually a monotone or with slightly varying tones. There are other similar sounds not produced by human singing that are also called hums, such as a sound produced by machinery in operation or by an insect in flight. The hummingbird was named for the sound that bird makes in flight.

Hummer is used as slang for a form of oral sex, where the giving partner makes a humming sound during the act, to add a source of vibration. Frank Zappa used hum as a pun in his song, Dinah-Moe Humm, with the reference being both to this slang usage and the hum made by a dynamo in operation.

Hum is British slang for stink. For example, "That is humming," would mean "That smells bad." Humming can also be attributed to being bored. Often, people who hum are bored and want you to know it, so they hum. Humming can also be a sign of being in an uncomfortable situation, so out of nervousness, a person might start humming

Look up hum in
Wiktionary, the free dictionary.

• The Hum - an apparently widespread phenomenon involving a low-frequency hum of unknown origin, inaudible to most people
• Mains hum - an electric or electromagnetic phenomenon that causes a low frequency (50 or 60 Hz) audible signal.