1/8-inch stereo plug

This is probably the type of audio plug that people in general will come into contact with most frequently-- you've almost certainly got at least one device somewhere in your house that uses one of these. It's the "small" kind, the kind that you see on almost all headphones, the kind you plug in to all walkmen, and the kind you plug in to almost all computers for audio-out and (usually) audio-in.

You sometimes see this plug type going by the name "1/8-inch stereo miniplug" or sometimes even "1/8-inch mini-stereo plug". Outside of the U.S. you would probably see "3.5 mm" in the name in place of "1/8 inch".

It's important to note that stereo plugs (2-channel) are distinguishable from mono plugs (1-channel) by the number of black stripes on the end of the plug itself. If it has a thick and thin line (thicker towards the end), it's stereo. If it has only one line, it's mono.

If you want to bring a source from stereo to mono, don't try to use a mono plug, because you will often only get one of the two channels (for instance, only left), instead of a mix of both. Instead, use a specialized adapter, something you can probably find at Radio Shack or some similar electronics or audio hardware store.