Definition of "Hum" :
noun: a humming noise
"The hum of distant traffic."
noun: 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
noun: the state of being or appearing to be actively engaged in an activity
"There is a constant hum of military preparation."
verb: sing with closed lips
"She hummed a melody."
verb: make a low continuous sound
"The refrigerator is humming."
verb: sound with a monotonous hum
verb: be noisy with activity