Definition of "Activity" :

noun: any specific behavior

"They avoided all recreational activity."

noun: the trait of being active; moving or acting rapidly and energetically

"The level of activity declines with age."

noun: an organic process that takes place in the body

"Respiratory activity."

noun: a process existing in or produced by nature (rather than by the intent of human beings)

"Volcanic activity."

noun: the state of being active

"His sphere of activity."

noun: (chemistry) the capacity of a substance to take part in a chemical reaction

"Catalytic activity."