when you stoke a string with alternate down strokes and up stokes for a real fast picking speed

Like Up, Down, Up, Down, on a string as fast as you can.

The name comes from the classical guitar technique Tremolo which is playing the strings just like that way, but finger picking with you Index, Middle, and Ring fingers. Then you play the bass with your thumb.

here:http://www.guitartricks.com/freelesson.php?input=YDI7ZDhkYTE=
thats temolo picking