term:

Testimonium Clause

testimonium clause n

[Latin testimonium testimony]
: the authenticating clause of an instrument (as a deed) that typically begins “In witness whereof” and furnishes such information as when it was signed and before what witnesses

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.