1. [n]  the act of singing with closed lips
2. [n]  a humming noise; "the hum of distant traffic"

Humming
Humming

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1. \Hum"ming\, a.
   Emitting a murmuring sound; droning; murmuring; buzzing.
   

2. \Hum"ming\, n.
   A sound like that made by bees; a low, murmuring sound; a
   hum.
   {Hummingale}, lively or strong ale. --Dryden.
   {Humming bird} (Zo["o]l.), any bird of the family
      {Trochilid[ae]}, of which over one hundred genera are
      known, including about four hundred species. They are
      found only in America and are most abundant in the
      tropics. They are mostly of very small size, and are not
      for their very brilliant colors and peculiar habit of
      hovering about flowers while vibrating their wings very
      rapidly with a humming noise. They feed both upon the
      nectar of flowers and upon small insects. The common
      humming bird or ruby-throat of the Eastern United States
      is {Trochilus culubris}. Several other species are found
      in the Western United States. See {Calliope}, and
      {Ruby-throat}.
   {Humming-bird moth} (Zo["o]l.), a hawk moth. See {Hawk moth},
      under {Hawk}, the bird.