What is Accessory After The Fact?

Legal Definition
Someone who assists another 1) who has committed a felony, 2) after the person has committed the felony, 3) with knowledge that the person committed the felony, and 4) with the intent to help the person avoid arrest or punishment. An accessory after the fact may be held liable for, inter alia, obstruction of justice.
Illustrative case law
See, e.g. Com. v. Dalton, 259 Va. 249 (2000).
See also
Legal Definition
One who knowing that another has committed a felony, relieves, comforts or assists him. See 26 Fed. Cas. (U. S.) 196.
-- Ballentine's Law Dictionary
Legal Definition
A party who commits a crime by hiding the offender or aiding in their escape preventing punishment.