When something is interdenominational, it brings together Christians who belong to different church families — Baptists, Catholics, Presbyterians, Methodists, and so on. A denomination here is not money; it is a named group of churches that share a way of worship and a set of beliefs. An interdenominational prayer meeting, school, or charity is not owned by one church family alone. People usually keep their own church; they just work or pray together for a time. Though actually, the word stays inside Christianity. If Buddhists, Muslims, and Christians meet, that is a different word: interfaith. You will hear interdenominational most in church notices, Christian schools, and news about religion. It is a long, rather formal word. In everyday talk people often just say “from different churches.”