Mostly means for the greater part or usually, but not completely or always. When something is mostly true, it's true most of the time but has some exceptions. If your homework is mostly finished, you've done the bulk of it but still have a bit left. When the weather forecast says it will be mostly sunny, expect plenty of sunshine with perhaps a few clouds drifting by.

The word helps us describe situations that aren't absolute. A student who is mostly honest might tell the truth nearly all the time but occasionally bend it. A movie that's mostly good might have a few slow parts. Your room might be mostly clean even if a few books are still scattered on the floor.

Mostly is useful because life rarely deals in absolutes. It acknowledges that things can be generally one way while leaving room for exceptions. When your teacher says the class did mostly well on a test, she means the majority succeeded, though perhaps a few students struggled. The word carries a sense of “the big picture” or “on the whole,” helping us describe reality more accurately than words like “always” or “completely” do.