What is the term for those cylindrical metal or plastic protrusions?

What is the term for those cylindrical metal or plastic protrusions that are inserted into mounting holes to mount a thing onto another thing? At first I thought of tab, but to me that makes it sound like it's flat, not cylindrical. Is there a proper term, or is it tab after all?

So far I've tried

protrusion, projection, lug, tab, knob, and nub

but none of these seem to be right.

This picture is not exactly what I had in mind, but it is close. Instead of a "rotated T" shape, picture a regular cylinder used to mount or align things to be mounted. I knew about dowels, but I thought that word could only be used when the pieces were separate like in Andrew's first two pictures. Are they still called dowels when they are one with the object being mounted?

Pegs which are used to locate parts together are generally called dowels:

They can be wood or metal, and can be used with glue (or slightly oversize) to actually effect a solid joint, or simply used to ensure that the parts are located correctly prior to being secured by other means.

I have furniture which is put together with wooden dowels just like the first picture and simply glued; but the cylinder head on my car engine is located with metal dowels similar to those in the second picture and then bolted to the block.

There is also the tenon which fits into a hole called a mortise. Generally these are rectangular, but could be turned and drilled respectively to provide a cylindrical fitting. The difference from a dowel is that the tenon is formed from the end of the piece it's part of.

A simple projection is called a lug:

In this photo, the two arrowed lugs have sheared, but the word describes both them (which resemble what you describe) and the long bolt-like lugs which are threaded to accept nuts.

I'd refer to the projecting element in the question's picture as a boss, in sense 4: "(mechanics) A protrusion, frequently a cylinder of material that extends beyond a hole." This is the sense of boss as used in reference to a similar picture in an Eagle Group pdf document that says:

Install drawer/shelf supports by engaging rear mounting boss into slot at rear of cart and engage front mounting boss into keyhole slot at front of cart.