Source : Webster's Revised Unabridged Dictionary (1913)

Impellent \Im*pel"lent\, a. [L. impellens, p. pr. of impellere.]
   Having the quality of impelling.

Impellent \Im*pel"lent\, n.
   An impelling power or force. --Glanvill.

Source : WordNet®

impellent
     adj : forcing forward or onward; impelling; "an impellent power";
           "an impellent cause"