Black's Law Dictionary, Sixth Edition, p. 261:

Collateral attack.  With respect to a judicial proceeding, an attempt to avoid, defeat, or evade it, or deny its force and effect, in some incidental proceeding not provided by law for the express purpose of attacking it.  May v. Casker, 188 Okl. 448, 110 P.2d 287, 289.  An attack on a judgment in any manner other than by action or proceeding, whose very purpose is to impeach or overturn the judgment; or, stated affirmatively, a collateral attack on a judgment is an attack made by or in an action or proceeding that has an independent purpose other than impeaching or overturning the judgment.  Travis v. Travis' Estate, 79 Wyo. 329, 334 P.2d 508, 510.  Compare Direct Attack.

[Black's Law Dictionary, Sixth Edition, p. 261]