A miracle is something that is believed to have 3 basic qualities. A miracle is something that breaks the laws of nature, something that has purpose and significance and also something that religious significance can be ascribed to.
Paul Tillich said that a miracle is an astonishing event that points to the mystery of being.
Aquinas:
Aquinas believed that miracles fell into 3 categories. A miracle is an event done by God that is something that nature itself could never do. An example of this is stopping the sun. Also, a miracle is an event done by God but something that nature could not do in that order. An example of this is an exorcism. Finally, a miracle is something that God and nature can do, but God can do without the use of natural laws.
Swinburne:
Swinburne classes a miracle as something that seems apparently impossible.
This is because the laws of nature are reasonably predictable. For example, a man coming to life after 24 hours of being presumed dead can be classed as a miracle. Swinburne also believes that there must be a reason for God to intervene in our universe, showing that it must have deep religious significance.
Swinburne believed in the principle of testimony and credulity. By this he means that people would tell the truth as there is no reason for them to lie.
Therefore, we should assume that people are telling the truth. This is therefore a key reason for believing in miracles.