A siddur is a Jewish prayer book containing a set order of daily prayers. The word siddur comes from the Hebrew root ס־ד־ר‎, meaning 'order.'

A prayer book is a book containing prayers and perhaps devotional readings, for private or communal use, or in some cases, outlining the liturgy of religious services. Books containing mainly orders of religious services, or readings for them are termed "service books" or "liturgical books", and are thus not prayer-books in the strictest sense, but the term is often used very loosely. A religion's scriptures might also be considered prayer books as well.