Miracle plays, or Saint’s plays, are now distinguished from mystery plays as they specifically re-enacted miraculous interventions by the saints, particularly St. Nicholas or St. Mary, into the lives of ordinary people, rather than biblical events; however both of these terms are more commonly used by modern scholars than they were by medieval people, who used a wide variety of terminology to refer to their dramatic performances. Robert Chambers, writing in 19th century notes that “especially in England, miracle [came] to stand for religious play in general”.
Encyclopedia.com Entry on Miracle Play.
New Advent Catholic Encyclopedia Very good account of Medieval religious drama, inlcuding miracle plays.
The Lit Cafe Brief definition of Miracle play.