What is Animus Testandi?

Legal Definition
An intention to make a testament or will. This is required to make a valid will; for whatever form may have been adopted, if there was no animus testandi, there can be no will. An idiot for example, can make no will, because he has no intention.
-- Bouviers Law Dictionary
Legal Definition
An intention to make a testament or will. Farr v. Thompson, 1 Speers (S. C.) 105. Animus ad, se omne jus ducit. It is to the intention that all law applies. Law always regards the intention. Animus hominis est anima scripti. The intention of the party is the soul of the instrument. 3 Bulst. 67; -Pitm. Prin. & Sur. 26. In order to give life or effect to an instrument, it is essential to look to the intention of the individual who executed it
-- Black's Law Dictionary