term:

Mansfield Rule

mansfield rule n

often cap R
[after William Murray, first Earl of Mansfield (1705-1793), British jurist who set forth the rule]
: a rule that a juror's affidavit or testimony as to juror misconduct during deliberations may not be used to impeach the verdict

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