What is the meaning of hum?

noun

• The state of being or appearing to be actively engaged in an activity.
  • usage: "they manifested all the busyness of a pack of beavers"; "there is a constant hum of military preparation"
  • synonyms: busyness
• An Islamic fundamentalist group in Pakistan that fought the Soviet Union in Afghanistan in the 1980s; now operates as a terrorist organization primarily in Kashmir and seeks Kashmir's accession by Pakistan.
  • synonyms: Harkat-ul-Mujahidin, Harkat ul-Ansar, HUA, Harkat ul-Mujahedeen, Al Faran, Movement of Holy Warriors
• A humming noise.
  • usage: "the hum of distant traffic"
  • synonyms: humming

verb

• Sing with closed lips.
  • usage: "She hummed a melody"
• Be noisy with activity.
  • usage: "This office is buzzing with activity"
  • synonyms: buzz, seethe
• Sound with a monotonous hum.
  • synonyms: thrum
• Make a low continuous sound.
  • usage: "The refrigerator is humming"