The judge will instruct them on the legal definition of a reasonable doubt.
Here is how some courts define it:
Under the law, the accused is presumed to be innocent of the offense(s). The government has the burden of proving the accused’s guilt by legal and competent evidence beyond a reasonable doubt.
1) A “reasonable doubt” is an honest, conscientious doubt suggested by the material evidence or lack of it in the case. It is an honest misgiving generated by insufficiency of proof of guilt. “Proof beyond a reasonable doubt” means proof to an evidentiary certainty, although not necessarily to an absolute or mathematical certainty. The proof must exclude every fair and reasonable hypothesis of the evidence except that of guilt.
2) A “reasonable doubt” is a conscientious doubt, based upon reason and common sense, and arising from the state of the evidence. Some of you may have served as jurors in civil cases, or as members of an administrative board, where you were told that it is only necessary to prove that a fact is more likely true than not true. In criminal cases, the government's proof must be more powerful than that. It must be beyond a reasonable doubt. Proof beyond a reasonable doubt is proof that leaves you firmly convinced of the accused's guilt. There are very few things in this world that we know with absolute certainty, and in criminal cases the law does not require proof that overcomes every possible doubt. If, based on your consideration of the evidence, you are firmly convinced that the accused is guilty of the offense charged, you must find (him) (her) guilty. If, on the other hand, you think there is a real possibility that the accused is not guilty, you must give (him) (her) the benefit of the doubt and find (him) (her) not guilty.
3) By “reasonable doubt” is intended not a fanciful, speculative, or ingenious doubt or conjecture, but an honest and actual doubt suggested by the material evidence or lack of it in the case. It is a genuine misgiving caused by insufficiency of proof of guilt. Reasonable doubt is a fair and rational doubt based upon reason and common sense and arising from the state of the evidence. Proof beyond a reasonable doubt is proof that leaves you firmly convinced of the accused's guilt. There are very few things in this world that we know with absolute certainty, and in criminal cases, the law does not require proof that overcomes every possible doubt.