Don’t know if there is any official “Catholic” definition…but I’d say it is something that surpasses human comprehension or human ability, taking place without any human intervention that we could easily recognize/calculate. For example, a person with some cancer prays/is prayed for and at a subsequent doctor visit the cancer is “gone”, despite having not been treated by the doctors. This would be something that defies our understanding of the nature of cancer and our understanding of what it takes to heal cancer…and happened without human intervention (unless prayer counts as intervention). It would be considered a miracle…and act of God.