A low, continuous sound, often made by machinery or electronics. The hum of the refrigerator was barely noticeable in the quiet kitchen.
A state of activity or excitement. The office was a hum of activity as the team prepared for the big presentation.
A vocal sound made with closed lips, often as a tune or melody. She absentmindedly let out a hum as she worked on her painting.
(informal) A strong, unpleasant smell. There was a hum coming from the rubbish bins in the heat.