One who abets; an instigator of an offense or an offender.

one who helps or encourages or incites another

a person who, being present or in the neighborhood, incites another to commit a crime, and thus becomes a principal in the offense

one who encourages or incites, persuades or sets another on to commit a crime. Such a person is either a principal or, an accessory to the crime. When present, aiding, where a felony is committed, he is guilty as principal in the second degree ; when absent, "he is merely an accessory." 1. Russell, 21; 1 Leach 66; Foster 428. Source: Bouvier's Law Dictionary, Sixth Edition, Revised, 1856.

Abettor (from to abet, O. Fr. abeter, a and beter, to bait, urge dogs upon any one; this word is probably of Scandinavian origin, meaning to cause to bite), is a law term implying one who instigates, encourages or assists another to commit an offence.