A scene is a unit of story that takes place at a specific location and time. If one of these changes, you have a new scene.
This is two scenes:
INT. PETE’S ROOM – NIGHT
He turns off the lamp and quietly nestles himself under the covers...
INT. PETE’S ROOM – DAY
A SCREECHING alarm wakes him abruptly.
This is also two scenes:
EXT. BACKYARD – NIGHT
Mask pulled down, Steve inserts the pick into the lock. He turns the tension wrench and they enter...
INT. LAUNDRY ROOM – CONTINUOUS
They creep quietly, cat-like.
The specifics of the definition can vary slightly from person to person, so make sure you get clarification if necessary. If, for example, an executive is giving you notes on the above and says, “Lose this scene, but not the one after it,” you should be sure you’re on the same page.