Dictionary.net

A requirement for anyone making a legal document, such as a will or healthcare directive. For example, although he can be eccentric or forgetful, a person writing a will must know what he owns, the identities of his family and close friends, and how the will distributes his property. If a person isn't of sound mind, and someone later challenges the validity of the document in a lawsuit, the judge could rule that the document is invalid and has no legal effect. (Such lawsuits are quite rare.)