It's not hard to find dictionary definitions of 'beyond reasonable doubt'. For example, see here:
http://legal-dictionary.thefreedictionary.com/ beyond%20a%20reasonable%20doubt
However, in practice, each juror will have their own interpretation of the phrase, so it will always be hard to attain true consistency.
I've done a bit of court reporting (and I sometimes visit the Crown court just out of curiosity). I recall one judge who always gave the jury the same advice which, in my opinion, summarised the concept of 'reasonable doubt' better than any text book can do. It went roughly like this:
"Members of the jury, the prosecution might convince you that it's
possible that the defendant committed this offence. If so, you must return a verdict of 'not guilty'. The prosecution might convince you that it's
likely that the defendant committed this offence. If so, you must return a verdict of 'not guilty'. The prosecution might convince you that it's
almost certain that the defendant committed this offence. If so, you must return a verdict of 'not guilty'. If, and only if, the prosecution can convince you that the defendant
did commit this offence, and that
no other conclusion could be reached by any reasonable person you may
consider a verdict of 'guilty' but, in making such consideration, you should repeatedly ask yourselves whether you've examined all of the evidence to the contrary".
Chris