The notion that both parties are equally bound or not bound to something. One party cannot be bound while the other is not. Black’s Law Dictionary 2nd ED.
Both parties must be bound to perform or the law will treat the agreement as if neither party is bound to perform.
I hire Ed to move my lawn and agree to pay him $30.00 for mowing the law. Ed mows the lawn and now I have to pay him the $30.00