What is the doctrine of collateral attack?

The doctrine of collateral attack was described by McIntyre J. in Wilson v. The Queen, [1983] 2 SCR 594, 1983 CanLII 35 (SCC) at S.C.R. p. 599 as follows: It has long been a fundamental rule that a court order, made by a court having jurisdiction to make it, stands and is binding and conclusive unless it is set aside on appeal or lawfully quashed. It is also well settled in the authorities that such an order may not be attacked collaterally—and a collateral attack may be described as an attack made in proceedings other than those whose specific object is the reversal, variation, or nullification of the order or judgment.