What is the legal test for defining the concept of "beyond a reasonable doubt"?

The courts, recognizing the difficulty and peril inherent in such a task, have discouraged "experiments" by trial courts in defining the "beyond a reasonable doubt" standard. (See People v. Freeman, supra, 8 Cal.4th 450, 503, 504, 34 Cal.Rptr.2d 558, 882 P.2d 249.) By a parity of reasoning, similar perils undoubtedly would attend a prosecutor's attempt to reduce the concept of guilt beyond a reasonable doubt to a mere line on a graph or chart. But as we have previously observed, errors or misconduct occurring [906 P.2d 31] during jury voir dire, prior to the introduction of evidence or the giving of formal instructions, are far less likely to have prejudiced the defendant.