activity vs hum

activity and hum both are nouns.

Word	Noun	Adjective	Verb	Adverb
activity	Yes	No	No	No
hum	Yes	No	Yes	No

As nouns, hum is a hyponym of activity; that is, hum is a word with a more specific, narrower meaning than activity:

activity (noun)	hum (noun)
any specific behavior	a humming noise
the trait of being active; moving or acting rapidly and energetically	the state of being or appearing to be actively engaged in an activity
an organic process that takes place in the body
a process existing in or produced by nature (rather than by the intent of human beings)
the state of being active
(chemistry) the capacity of a substance to take part in a chemical reaction