Dictionary.net

A party's job of convincing the decisionmaker in a trial that the party's version of the facts is true. In a civil trial, it means that the plaintiff must convince the judge or jury "by a preponderance of the evidence" that the plaintiff's version is true -- that is, over 50% of the believable evidence is in the plaintiff's favor. In a criminal case, because a person's liberty is at stake, the government has a harder job, and must convince the judge or jury beyond a reasonable doubt that the defendant is guilty.