What is an Act?

If you read books about playwriting, you do not encounter any particular obsession about how many "acts" a play should have. Racine wrote plays in 5 acts. Contemporary playwrights tend to write 1-act or 2-act plays.

In contemporary theater usage, you can tell a 1-act from a 2-act by noticing whether there is an intermission. As a matter of fact, for a 90-minute play, one theater may present it as a 1-act, with no breaks, and another theater might present it as a 2-act.

But screenwriting books are obsessed with 3-act structure, which at first seems odd, because movies no longer have intermissions. As it turns out, there's a simple explanation, because when they talk about the 3 acts, they are talking about beginning, middle, and end.

This line of thought has an ancestor:

A whole is that which has a beginning, a middle, and an end. A beginning is that which does not itself follow anything by causal necessity, but after which something naturally is or comes to be. An end, on the contrary, is that which itself naturally follows some other thing, either by necessity, or as a rule, but has nothing following it. A middle is that which follows something as some other thing follows it. A well constructed plot, therefore, must neither begin nor end at haphazard, but conform to these principles.

That's an Attic flashback from Aristotle.

The audience feels more satisfaction when they perceive a "whole action".

With beginning, middle, and end,
you hopefully will not send
the audience home feeling cheated
by a story that wasn't completed.