A mosque is a place of worship for Muslims, where they perform their obligatory prayers (salah) . The word "mosque" evolved from the Arabic term "masjid," which means "place of prostration". Mosques are usually covered buildings, but they can be any place where prayers are performed, including outdoor courtyards. The Prophets mosque introduced some of the features still common in todays mosques, including the niche at the front of the prayer space known as the mihrab and the tiered pulpit called the minbar. Mosques are centers of cities or neighborhoods in cities, and they are symbolically very important to Muslims. Mosques are not only places of worship but also centers of community and education. Muslims are permitted to pray anywhere, excluding filthy or impure places such as toilets or graveyards. Nonetheless, mosques provide facilities for spiritual advancement activities for children and adults, classes to be taught for religious education, and a center for discussion of propagation of Islam.