An action that takes place in a single setting. It's important that a scene has a beginning, middle and end, and that it forwards the story. Multiple scenes, belonging together, is a sequence.

A distinct beat of the story, in which a character starts in one emotional and mental condition, takes an action which has consequences, and ends up in a slightly different emotional and mental place...?

Scenes are defined by the action that is taking place - they include a slug line, some narrative description, and the dialogue. I strive to give each of my scenes a beginning, middle, and end, but don't necessarily begin at the beginning - I like to come in late and get out early. My scenes often are 1-3 pages in length. When I take a scene out of the context of the script, it still makes sense because it has its own story to tell.

These are all solid responses. Another way of looking at scenes: They are building blocks, each part of a whole. Every scene exists on its own merits, but is also tied to the overall narrative. Beginning, Middle and End? Yes. Enter late, exit early? Yes. 1-3 pages long? Actually the average length of a scene nowadays [broadly speaking] is 1 1/2 pages. Perhaps the single most important question you can ask of every scene is this: What is the point of the scene? If you can't answer that question, then either the scene is problematic, or you haven't dug deep enough into the narrative to figure out each scene.

A particular chunk of time in a particular place with a particular character or characters .. for a particular purpose in the overall story. As important as noting "what happens" on the proverbial index card for each scene is knowing "why" it needs to happen. Is this scene moving someone closer to their goal? Hampering them? Pitfall/roadblock? Lynch pin toward something? If there's no solid, compelling, story-driven reason "why" a scene is there, it doesn't need to be there .. and should politely go away.