Obtained OG / Potential OG. Say you mashed 3lbs of 2-row in 1gal, 100% efficiency would give you an OG of 1.108 (3*36/1). If you measured it to be 1.081, then you got 81/108 = 75% efficiency
If you have beer smith, go into the "recipe view". On the fourth line from the top, next to the "Help" button, there is a button labeled "Brewhouse Efficiency". Click it and it will bring up the screen for figuring it out.