We often hear terms like:  "forward"or "in front of the leg".  It is important to discuss the meaning and difference however. 
Impulsion is thrust and includes the idea of "desire to go"  along with a relaxation of the back.  Obviously a race horse has a desire to go forward but doesn't have the added quality of relaxation of the back that is necessary for a dressage horse.
A horse is "in front of the leg" if he is obedient to the driving aids.  We usually include the seat in this comment.
The bottom line is that we want our horses to respond very quickly to any driving aid but to do so with confidence.  This creates energy without tension . . .our ultimate goal.