In 1850, the Massachusetts Supreme Court, Commonwealth v. Webster, said reasonable doubt was a mental state in which a jurors "cannot say they feel an abiding conviction, to a moral certainty, of the truth of the charge."
In Cage v. Louisiana, 498 US 39 (1990), the US Supreme Court said what it wasn't. In that case, the high court rejected Louisiana's definition of reasonable doubt as "such doubt that would give rise to a grave uncertainty... A reasonable doubt is not a mere possible doubt. It is an actual substantial doubt. It is a doubt that a reasonable man can seriously entertain."
In Victor v. Nebraska, 511 US 1 (1994), Justice Ruth Bader Ginsberg defined proof beyond a reasonable doubt as "proof that leaves you firmly convinced of the defendant's guilt."
In Sandoval v. California, the US Supreme Court upheld the following definition of reasonable doubt:
"Reasonable doubt is defined as follows: It is not a mere possible doubt; because everything relating to human affairs, and depending on moral evidence, is open to some possible or imaginary doubt. It is that state of the case which, after the entire comparison and consideration of all the evidence,leaves the minds of the jurors in that condition that they cannot say they feel an abiding conviction, to a moral certainty, of the truth of the charge."
The term is not defined as part of the jury charge in Texas courts. Some folks have described beyond a reasonable doubt as that degree of certainty upon which you make your most important decisions.
In 48 states, a jury must return a unanimous verdict in order to convict a citizen of committing a criminal offense. While the requirement of unanimity is not required under the Constitution, it is a pretty good indicator that the state has proven its case beyond a reasonable doubt.
Louisiana and Oregon are the only two states that allow for a conviction upon a jury vote of 10-2 in a felony case. The equates to 5/6 of the panel - that leaves an awful lot of room for doubt, wouldn't you say?