Testamentary capacity is the ability to:

Testamentary capacity (testament = will) is the mental ability to make a valid will. Will denotes any testamentary document. The requirement for a valid will are as follows. A written and properly signed and witnessed document must exist. The testator must be a major and of sound disposing mind at the time of making the will. Force undue influence or dishonest representation of facts, should not have been applied by others. Ref: The Essentials of Forensic Medicine and Toxicology 29th Ed Page 431