"Outlet" is a generic term for all branch type connections. They can come with threaded, socket-weld, and butt-weld end connections. The most common types are used in place of a tee. A hole is cut in the side of the pipe, the outlet is welded in place over the hole and then the additional length of pipe is attached to the outlet.
Similar to reducing tees, outlets are usually described by two sizes. The Run or Header size refers to the pipe on which the outlet is mounted. The Outlet or Branch size refers to the length of pipe branching off of the run.
This type of fitting is contoured to match the curve on the Header Pipe and beveled on the Branch side to mate up with a beveled piece of the branch pipe.
In addition, various other types of outlets are available such as: 45 degree lateral outlets which branch out on a 45 degree angle, elbow outlets which sit on the radius of a 90 degree elbow and nipple outlets which combine a nipple length (usually 3 1/2" long) with an outlet.