Good cause is a legal term denoting adequate or substantial grounds or reason to take a certain action, or to fail to take an action prescribed by law. What constitutes a good cause is usually determined on a case by case basis and is thus relative.

Often the court or other legal body determines whether a particular fact or facts amount to a good cause. For example, if a party to a case has failed to take legal action before a particular statute of limitations

Statute of limitations

A statute of limitations is an enactment in a common law legal system that sets the maximum time after an event that legal proceedings based on that event may be initiated...

has expired, the court might decide that the said party preserves its rights nonetheless, since that party's serious illness is a good cause, or justification for having additional time to take the legal action.