Is there a clear definition of the reasonable doubt standard?

1. We note that a clear definition of the reasonable doubt standard has evaded even learned federal judges. See, e.g. , United States v. Adkins, 937 F.2d 947, 950 (4th Cir. 1991) ("This circuit has repeatedly warned against giving the jury definitions of reasonable doubt, because definitions tend to impermissibly lessen the burden of proof. . . . The only exception to our categorical disdain for definition is when the jury specifically requests it."), (internal citations omitted); United States v. Hall, 854 F.2d 1036, 1039 (7th Cir. 1988) (upholding the district court's refusal to provide a definition, despite jury's request, because "at best, definitions of reasonable doubt are unhelpful to a jury . . . . An attempt to define reasonable doubt presents a risk without any real benefit.).