Abuse of discretion describes situations in which a court exceeds legal boundaries in a case, depriving someone of the right to a fair trial in the process. In cases in which abuse of discretion has occurred, a review of the case by a higher court on appeal can result in a reversal of the ruling. It is extremely challenging to prove abuse of discretion, however, and usually it must be shown that the court acted so far out of bounds that it is simply indefensible.
Judges are expected to apply the law fairly and equitably, no matter who is before them in court. They are allowed some discretion, however. Judges rule on what kind of evidence should be admitted, including who is allowed to testify; deal with motions made during the case; and come up with a sentence if someone is convicted. While there are rules to govern all of these processes, there is some leeway to allow judges to consider the individual case at hand and make decisions which are fair.
If a court applies the wrong law or acts in a way which is clearly illegal, such as suppressing a witness who could have provided important information, this is abuse of discretion. Likewise, if a court makes a decision but cannot support the decision with evidence, or otherwise acts in a way which is not reasonable or sound, it may be considered an abuse of discretion.
It is not necessary for bad faith to be present in order to prove abuse of discretion. Sometimes it is an innocent error. A judge may not realize the importance of evidence, for example, or may not be familiar with a precedent which established a different way of approaching a given legal conundrum. Judges may also act on internalized prejudices and be unaware on a conscious level of the fact that they are not applying the law fairly.
Higher courts need to see clear evidence of abuse of discretion, such as unequal sentences applied for similar offenders or clear evidence that a witness was excluded from a case illegally. The higher court weighs the need to preserve judicial discretion with the equally important need to ensure that all people have access to a fair trial. If the lower court did indeed act in a way which is indicative of abuse of discretion, its ruling will be overturned and it will be necessary to hold a new trial.