My terminology is a bit lacking so please bear with me. The doors on my 2004 Toyota RAV4 are very loose and will only stay open if I'm parked on a flat or declined surface. If I remember correctly they used to have a "notch" where they would stay about half-way open and all of the way open. That notch feels like it's gone and the doors are completely at the mercy of wind and gravity. Is there something I can adjust to tighten them back up?
There are two different ways of doing that, so it depends which Toyota have used - Either there is a notch on the hinge itself, or it is part of the check strap.
The latter is probably easier to check - between the hinges there should be a bar that is hinged at one end and slides at the other. It's main purpose is to sopt the door opening more than it should, but they can also be used to hold the door - look at it and see if there is any notch or lump halfway along. If there is, it's likely that the fixed part is worn if it doesn't stay in either position.
If it is part of the hinges, it'll be cast into the hinge itself. You mention that the door is loose - is that in all directions? i.e. if you open the door and grip it firmly, can you move it up and down. If so, the hinges themselves are worn and will probably need replacing (very few are adjustable). This is often more common on driver's doors as they are used more than the others.
This is a common 2005-2009 Prius problem. Prius Chat gives a good explanation on how to change them. I recommend you don't get replacement parts from salvage yard. I did this and the door stops again wore out in a few months. It's a pain when the door keeps shutting on you while you unload groceries. I wish there was an affordable aftermarket solution because it's an easy DIY change.
The part you are inquiring about is called a door brake. Looked it up for my 07 matrix, left rear door and if worn out they are about 50 bucks. Have to remove door panel to change it. Then a punch to pop the pin out on car side and 10 mm socket to remove from door.