According to ExRx, a synergist is:
A muscle that assists another muscle to accomplish a movement.
For example, during the dumbbell curl, the biceps is the target and the brachialis and brachioradialis are synergists.
However, according to the NSCA's book Essentials of Strength Training and Conditioning (4th Edition)
A muscle is called a synergist when it assists indirectly in a movement. For example, the muscles that stabilize the scapula act as synergists during upper arm movement. Without these synergists, the muscles that move the upper arm (many of which originate on the scapula) would not be effective in bringing about this movement.
These definitions are obviously different.
Other websites seem to have the same or a similar definition as the NSCA.
Why is the definition on ExRx different?