Ajar means slightly open, not fully closed but not wide open either. When a door is ajar, there's a narrow gap between it and the frame: you can see a sliver of light through it or peek into the next room. A window left ajar lets in fresh air without being thrown wide open.

The word captures that in-between state. A door isn't ajar if it's shut tight, and it isn't ajar if it's standing wide open. It's specifically that partial opening, like when someone leaves their bedroom door ajar so they can hear what's happening downstairs, or when you leave the car door ajar by accident and the warning bell starts dinging.

You'll mostly hear ajar used with doors, gates, and windows, though occasionally with other things that open and close like lids or hatches. The word has a slightly formal quality to it: in casual conversation, people might just say “the door's open a crack” or “leave it open a bit,” but ajar gives you that precise meaning in a single word.