In basketball, effective field goal percentage (abbreviated eFG%) is a statistic that adjusts field goal percentage to account for the fact that three-point field goals count for three points while field goals only count for two points.  Its goal is to show what field goal percentage a two-point shooter would have to shoot at to match the output of a player who also shoots three-pointers. 
It is calculated by:
It can also be calculated by:
The advantage of this second formula is that it highlights the aforementioned logic behind the statistic, where it is pretended that a player only shot two-point shots (hence the division of non-free-throw points by 2).
An additional formula that seems to be more in use by the statistics actually displayed on websites (but less cited by said websites) is:
All three equations yield the same result.