William P. Loewe of The Catholic University of America offers a fairly good discussion of miracles in his work on Christology.
"What is meant by a miracle? Many people when they hear the word think of something unexplainable or something that violates the laws of nature. The key to this common understanding of miracle lies in the question of causality; either/or characterizes the approach to this understanding of miracle- either there is some natural explanation or God did it. On this understanding something is a miracle if cannot be explained. "Cannot," however, is a tall order. There is a large difference between saying that we do not in fact understand something and saying that we cannot, in principle, ever come to understand it. The history of science itself suggests that we should exercise a good deal of modesty in making the second kind of statement...."
(Rather garbled image from the Shrine of the Immaculate Conception in Washington DC, on the campus of CU; the image of Christ in Glory is in the Apse. It's my favorite.)