Hi! Wanted to discuss this term and options for interpreting. Comments welcome!
Here are some links for definitions of "collateral attack":

collateral attack
n. a legal action to challenge a ruling in another case. For example, Joe Parenti has been ordered to pay child support in adivorce case, but he then files another lawsuit trying to prove a claim that he is not the father of the child. A "direct attack"would have been to raise the issue of parenthood in the divorce action.
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What is a collateral attack?
A collateral attack upon a conviction or sentence is broadly defined as any attempt to overturn the conviction or sentence that is not made in a direct appeal. A direct appeal takes place in a higher court. The appellate court reviews the proceedings in the trial court to decide whether any errors were made that entitle you to a new trial, a new sentencing hearing, or other relief. A collateral attack, on the other hand, is typically presented to the court in which you were convicted, often after a direct appeal has ended. A federal challenge to a federal conviction madein a 2255 motion, or to a state conviction in a 2254 motion, is a collateral attack upon the conviction.
Whether a challenge to a conviction is direct or collateral is usually, but not always, clear. Sometimes (although not often in federal court) a post-conviction motion can be made to the trial court before a direct appeal is filed. Depending on the nature of the motion and the applicable rules of criminal procedure, that motion might be considered a direct attack upon the conviction. A challenge to the conviction that is made after a direct appeal has ended will almost always be regarded as a collateral attack.
The law draws many distinctions between a challenge to your conviction made in a direct appeal and a challenge made in a collateral attack. The precise distinctions depend upon the state or federal laws that govern the court in which you were convicted. As a general rule, however, a collateral attack upon a conviction is more limited than a direct attack. A direct appeal can usually raise any claim of error that affected the fairness or outcome of a criminal proceeding. The defendant can base a direct attack upon errors made at any point in the criminal proceedings. It can be used to challenge rulings on pretrial motions, bias in the selection of the jury, the erroneous admission of evidence, improper statements and arguments made by the lawyers during the trial, flawed jury instructions, mistakes made in the imposition of sentence, and any other prejudicial error that might entitle a defendant to a new trial or a reduced sentence.
Unlike a direct appeal, a collateral attack is usually limited to claims of constitutional or fundamental error. The distinction is important although it is sometimes elusive. For instance, a successful claim that the court improperly admitted irrelevant evidence might lead to a reversal of a conviction if it is raised in a direct appeal, but might be rejected if it is raised in a collateral attack because the error did not violate a constitutional right. Although almost any error can be characterized as a violation of the constitutional right to a fair trial, courts do not always agree with that characterization. It is important to raise every meritorious issue that can be considered in a direct appeal because not all of those issues can be raised in a collateral attack.
In some jurisdictions, including federal court, challenges to a conviction based on new facts can only be raised in a collateral attack. New facts are any facts that are not included in the court record. Because new facts can persuade a court that your conviction or sentence was unjust, a collateral attack can be just as important as a direct appeal. For instance, if you were convicted because your lawyer made serious mistakes during your trial, a challenge based on ineffective assistance of counsel might persuade the court to vacate your conviction and give you a new trial. If you were convicted in federal court, a collateral attack is usually the only way to raise that issue because it will almost always involve new facts.
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