Single Butted

A spoke that has a different thickness from one end to the other or in otherwords it slowely tapers off from head to thread.
Spokes undergo more stress at the head and thread areas so if they are thicker in those regions they should theoretically be stronger. By using a smaller diameter in the middle of the spoke where the strength isn't as critical some weight savings can be had.

For example: a single butted spoke could measure: 14g/15g/15g or 2.0/1.8/1.8mm (Head/Middle/Thread)