The Collaborative International Dictionary of English v.0.48:

Accessary \Ac*ces"sa*ry\ (#; 277), a.
   Accompanying, as a subordinate; additional; accessory; esp.,
   uniting in, or contributing to, a crime, but not as chief
   actor. See Accessory.
   [1913 Webster]

         To both their deaths thou shalt be accessary. --Shak.
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         Amongst many secondary and accessary causes that
         support monarchy, these are not of least reckoning.
                                                  --Milton.
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The Collaborative International Dictionary of English v.0.48:

Accessary \Ac*ces"sa*ry\ (277), n.; pl. Accessaries. [Cf.
   Accessory and LL. accessarius.] (Law)
   One who, not being present, contributes as an assistant or
   instigator to the commission of an offense.
   [1913 Webster]

   Accessary before the fact (Law), one who commands or
      counsels an offense, not being present at its commission.


   Accessary after the fact, one who, after an offense,
      assists or shelters the offender, not being present at the
      commission of the offense.
      [1913 Webster]

   Note: This word, as used in law, is spelt accessory by
         Blackstone and many others; but in this sense is spelt
         accessary by Bouvier, Burrill, Burns, Whishaw, Dane,
         and the Penny Cyclopedia; while in other senses it is
         spelt accessory. In recent text-books on criminal law
         the distinction is not preserved, the spelling being
         either accessary or accessory.
         [1913 Webster]

WordNet (r) 3.0 (2006):

accessary
    adj 1: aiding and abetting in a crime; "he was charged with
           being accessory to the crime" [syn: accessary,
           accessory]
    n 1: someone who helps another person commit a crime [syn:
         accessory, accessary]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ACCESSARY, criminal law. He who is not the chief actor in the perpetration
of the offence, nor present at its performance, but is some way concerned
therein, either before or after the fact committed.
     2. An accessary before the fact, is one who being absent at the time
of, the crime committed, yet procures, counsels, or commands another to
commit it. 1 Hale, P. C. 615. It is, proper to observe that when the act is
committed through the agency of a person who has no legal discretion nor a
will, as in the case of a child or an insane person, the incitor, though
absent when the crime was committed, will be considered, not an accessary,
for none can be accessary to the acts of a madman, but a principal in the
first degree. Fost. 340; 1 P. C. 118.
     3. An accessary after the fact, is one who knowing a felony to have
been committed, receives, relieves, comforts, or assists the felon. 4 Bl.
Com. 37.
     4. No one who is a principal (q.v.) can be an accessary.
     5. In certain crimes, there can be no accessaries; all who are
concerned are principals, whether they were present or absent at the time of
their commission. These are treason, and all offences below the degree of
felony.  1 Russ. 21, et seq.; 4 Bl. Com. 35 to 40; 1 Hale, P. C. 615; 1 Vin.
Abr. 113; Hawk. P. C. b. 2, c. 29, s. 16; such is the English Law.  But
whether it is law in the United States appears not to be determined as
regards the cases of persons assisting traitors. Serg. Const. Law, 382; 4
Cranch, R. 472, 501; United States v. Fries, Parnphl. 199.
     6. It is evident there can be no accessary when there is no principal;
if a principal in a transaction be not liable under our laws, no one can be
charged as a more accessary to him. 1 W.& M. 221.
     7. By the rules of the common law, accessaries cannot be tried without
their consent, before the principals. Foster, 360. The evils resulting from
this rule, are stated at length in the 8th vol. of Todd's Spencer, pp. 329,
330.