A more thorough explanation:

The middle burden of proof is a party's duty to prove a fact by clear and convincing evidence. This standard lies between the preponderance-of-the-evidence standard and the beyond-a-reasonable-doubt standard. It is a type of burden of proof, which is a party's duty to prove a disputed assertion or charge.

For example, in a civil case, the plaintiff has the burden of proving their case by a preponderance of the evidence, which means that it is more likely than not that their version of events is true. However, in some cases, such as cases involving fraud or defamation, the plaintiff may have to prove their case by clear and convincing evidence, which is a higher standard than preponderance of the evidence but lower than beyond a reasonable doubt.

Another example is in a criminal case, where the prosecution has the burden of proving the defendant's guilt beyond a reasonable doubt. However, in some states, if the defendant raises an affirmative defense, such as self-defense, they may have the middle burden of proving that defense by clear and convincing evidence.