A peleh is a wonder or a miracle. At the splitting of the Red Sea, God is praised "awesome in splendour, working peleh [wonder]" (Exodus 15:11).
When Manoah asks the name of the angel who has come to announce the conception of Samson, the angel refuses, claiming that his name is peli - unknowable (Judges 13:18). A peleh is a type of miracle that is beyond our understanding, as in Leviticus 17:8: "if a case is too baffling - yipaleh mimcha davar".
A related biblical use of peleh concerns oaths. The verbal expression for making an oath is lefaleh neder.(Leviticus 22:21). Rashi explains the meaning of lefaleh as "to separate through speech". An oath is, in a sense, a wonder, a creation of a new reality through words.
Rabbi Yitzchak Hutner in his masterful Pachad Yitzchak explains that the reason the verb lefaleh is used for oaths is that the ability to speak is miraculous. Speech is a product of the soul and the body working together. This co-operation is what sets humanity apart and is a peleh. Hence Maimonides called his volume on the Jewish laws regarding speech Sefer Hapliyah, the "Book of Wonder".