David Hume's definition of a miracle (refining, paraphrasing): X is a miracle if (and only if) not X is an even greater miracle.
Is this definition problematic in any way?
When anyone tells me, that he saw a dead man restored to life, I immediately consider with myself, whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened.... If the falsehood of his testimony would be more miraculous, than the event which he relates; then, and not till then, can he pretend to command my belief or opinion. ~David Hume