 Philosophers usually assume that a prayer is answered if and only if God brings about the thing requested in part because of the offering of the prayer, so that had the prayer not been offered, the thing in question would not have occurred. If you pray to God for rain tomorrow and it does rain tomorrow, this all by itself is not enough to say that your prayer for rain was effective—it must also be the case that God actually brought about the rain at least in part because of your prayer. If it would have rained anyway, without your prayer for rain, then it doesn’t seem that your prayer for rain was effective. So an effective prayer would be a prayer that made a difference by influencing God to act. (For more on this question, see Flint 1998, chapter 10, Davison 2017, chapter 2, Pruss 2013, Johnson 2020, and Davison 2022, chapter 3.)