Abetter, Abettor, n.
One who abets; an instigator of an offense or an offender.
Note: The form abettor is the legal term and also in general use.
Syn. -- Abettor, Accessory, Accomplice. These words denote different degrees of complicity in some deed or crime. An abettor is one who incites or encourages to the act, without sharing in its performance. An accessory supposes a principal offender. One who is neither the chief actor in an offense, nor present at its performance, but accedes to or becomes involved in its guilt, either by some previous or subsequent act, as of instigating, encouraging, aiding, or concealing, etc., is an accessory. An accomplice is one who participates in the commission of an offense, whether as principal or accessory. Thus in treason, there are no abettors or accessories, but all are held to be principals or accomplices.
Shades of meaning
Fernald has no entry of its own for abettor; he lists it under accessory.
Where he writes about it:
In law, an abettor (the general legal spelling) is always present, either actively or constructively, at the commission of the crime; an accessory never. — from accessory
If present, though only to stand outside and keep watch against surprise, one is an abettor, and not an accessory. — from accessory
At common law, an accessory implies a principal, and can not be convicted until after the conviction of the principal; the accomplice or abettor can be convicted as a principal. — from accessory
Accomplice and abettor have nearly the same meaning, but the former is the popular, the latter more distinctively the legal term. — from accessory