What is the test for defining the phrase "beyond a reasonable doubt"?

Many courts stress the dangers of attempting to define the phrase "beyond a reasonable doubt." "It is said that scholastic attempts to explain the meaning of such words, which are more easily understood than explained, are liable to lead such men as commonly make up our juries to think that the ordinary processes of reasoning, by which they are accustomed to come to conclusions in the ordinary affairs of life, are not suitable to the jury room in a criminal case, but that some other process of reasoning is to be adopted which they are to gather from the language of the trial judge, and that they are thereby really weakened in their ability to come to a just conclusion; that it would be better to leave them to exercise their own intelligence in regard to language so plain that it is not easy to make it plainer by explanation." (Buel v. State (1899) 104 Wis. 132, 152, 80 N.W. 78, 85).