Definitions for "EC50"
" Effect Concentration 50% " or concentration having 50% of effect compared to control
The EC50 represents the plasma concentration/AUC required for obtaining 50 percent of the maximum effect in vivo.
In a dose-response relationship showing sublethal effects, the EC50 is the concentration that produces a level of effect = 50% of the maximum effect.  For example, the EC50 may be the concentration that causes a particular behavioral effect in 50% of the organisms that are tested. [compare to LC50