E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Accessory before the fact is one who is aware that another intends to commit an offence, but is himself absent when the offence is perpetrated.
Accessory after the fact is one who screens a felon, aids him in eluding justice, or helps him in any way to profit by his crime. Thus, the receiver of stolen goods, knowing or even suspecting them to be stolen, is an accessory ex post facto.