Activity; action or operation of a person, organization, animal, volcano, etc. Can be used as a noun or with 'suru' as a verb.
ボランティア活動に参加して地域の清掃を手伝った。
I joined volunteer activities and helped clean the neighborhood.
地震の後、火山活動が活発化したと報告された。
After the earthquake, it was reported that volcanic activity intensified.
彼は研究活動を通して多くの論文を発表した。
He published many papers through his research activities.
An obsolete/abbreviated term for 'katsudō shashin' (moving pictures / early film); rarely used today.
20世紀初頭、『活動』という語は活動写真（映画）を指すことがあった。
In the early 20th century, the word 'katsudō' could refer to moving pictures (films).