A miracle is anything unexplainable which seems to defy the laws of physics, nature, or logic in order to achieve an end, usually something very good or mystifying.
The more common definition of "miracle" is much looser than the technical definition. In the common definition a miracle is anything that causes wonder or amazement, even if it's completely explainable. For example, here's a comparison between the two definitions:
- Technical: "I was walking down the street and an iron girder suspended above a construction site fell as I was walking underneath. Right as it was about to hit me it suddenly flew to the left and landed in the middle of the street, completely missing me and everyone else there! God was watching out for me."
- Common: "My son was just born this morning. He has the most beautiful eyes in the world. Childbirth is such a miracle!"
Some Christians have a skewed and often disturbing view of what a miracle is. To them, a miracle is something which spares them pain or hardship, even if it deals out said pain or hardship on someone else. For example, a Christian man is driving down the street when a car comes out of nowhere and hits him. Both cars are totalled and everyone dies except him. Later on, when he's interviewed by the local news station he claims that God was watching out for him. Obviously this is total bunk. The Christian God is said to be omnipotent and omniscient, thus he knew the accident would happen, he knew how to save everyone, and he was able to. Why he didn't is anyone's guess. In any case, the fact that one man survived doesn't mean that he owes his continued existence to God. This is also a good example of the problem of evil.
In his book, Unweaving the Rainbow, Richard Dawkins coined a term to deal with "miracles" that merely turn out to be coincidental in nature: PETWHAC, or "Population of Events That Would Have Appeared Coincidental". Dawkins points out that many mundane events occur every single day to every person on the planet, and the human mind has a great capacity to identify patterns where none exist. Dawkins explains that there is a space of events with perfectly normal probability, which could be interpreted as "miraculous" by people who have that sort of mindset.
For instance, suppose that you are thinking about a friend, and that friend just happens to call you. Is this event supernatural in nature? Extremely doubtful. People think about others all the time, and they also call each other all the time. Given enough time, there is a virtual guarantee that a coincidence of this nature will occur.