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