1A character/element meaning life, living, or vitality. As a standalone noun it's somewhat literary; it's commonly used as a morpheme in compounds.
漢字の「活」は「活動」や「活力」のように使われることが多い。
The character '活' is often used in words like '活動' or '活力'.
この言葉では『活』が『活動』の略として理解される。
In this word, '活' is understood as an abbreviation of 'activity'.
2A (specialized/older) term for a resuscitation or enlivening technique in judo; a technical or archaic usage.
古い柔道書には、相手に活を与える技についての記述がある。
Some old judo texts describe techniques for restoring consciousness to an opponent.
3A suffix (abbreviated) meaning activity — used in compounds like 就活 (job-hunting) or 婚活 (marriage-hunting).
大学生の就活は忙しい時期だ。
Job-hunting is a busy time for university students.
婚活アプリを試してみる人が増えている。
More people are trying matchmaking apps.
サークル活にも力を入れている。
They're also putting effort into club activities.