Act is therefore single, individual, momentary; action a complex of acts, or a process, state, or habit of exerting power. We say a virtuous act, but rather a virtuous course of action.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

act Say what you will, I will act on my own judgement.
movement The address was in favor of their social movement.
activity Hey, let's make brushing our teeth a fun activity!
exercise People living in a big city tend to lack exercise.
case What number should I call in case of an emergency?
lawsuit If we win this lawsuit, it will become precedent.

Popularity over time

Word	1850 → 2019	Trend
action		falling
act		falling
movement		steady
activity		steady
exercise		steady
case		steady

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: act¹, sue¹
4 letters: work¹, play¹, case¹, turn¹, step¹, suit¹, plot¹, deed¹
5 letters: thing¹, doing², cause¹, force¹, fight¹, brush¹, fable², verve¹
6 letters: energy³, agency³, battle², acting², motion², combat², affair², fulfil²
7 letters: process², subject², measure², conduct², contest², lawsuit², execute³, gesture², fulfill²
8 letters: practice², movement², activity⁴, exercise³, behavior³, conflict², practise², skirmish², litigate³
9 letters: operation⁴, influence³, procedure³, behaviour³, encounter³, rencontre
10 letters: performing³, engagement³, accomplish³, litigation⁴, proceeding³, rencounter
11 letters: performance³, achievement³, transaction³, prosecution⁴
14 letters: representation⁵

The small number after each word is its number of syllables (doing² = 2 syllables).

In other languages

Français: action
Italiano: azione
Español: acción
Deutsch: Einfluss
Català: acció