Indirect evidence that implies something occurred but doesn't directly prove it. If a man accused of embezzling money from his company had made several big-ticket purchases in cash around the time of the alleged embezzlement, that would be circumstantial evidence that he had stolen the money.
Evidence which may allow a judge or jury to deduce a certain fact from other facts which have been proven. In some cases, there may be evidence essential to prove a case that cannot be proven directly, such as with an eye-witness. In these cases, the lawyer will provide the judge or juror with evidence of the circumstances from which a juror or judge can logically deduct, or reasonably infer, the fact that cannot be proven directly; it is proven by the evidence of the circumstances; hence, "circumstantial" evidence. Fingerprints are an example of circumstantial evidence: while there may be no witness to a person's presence in a certain place, or contact with a certain object, the scientific evidence of someone's fingerprints is persuasive proof of a person's presence or contact with an object.