Under Christianity and Judaism, God is truth and a miracle is nothing more than a "sign" of the truth. "Miracle" is the non-believer's term for what believers properly describe as a "sign", a term that does appear repeatedly in the Bible.
A purely scientific definition of a miracle is this: a net decrease in [[entropy]]. It follows from this definition that the greatest miracle would be the conversion of decaying matter to human life, as in the Resurrection.