2.1mm barrel power jack socket. For connecting barrels, I mean "power leads"!
Look, it's a Jack! Not to be confused with a Mike, Tom, or Harry! And this isn't any Jack, but a Power Jack. Think "CEO" or "Mega Businessman". Or maybe it's just a Barrel Jack socket! In that case, I hope it's not like one of those old fashioned homeless people wearing nothing but a barrel. That'd be creepy.
Witty copywriter comments aside, this is a 2.1mm center-pin barrel jack socket, with 2 conductors and 3 contacts (internally normally closed). It's much like any other standard DC jack you've seen, exactly like the ones we use on our own SB-Freeduino.
Note: This item is sold individually, and not in pairs.