The ex is pronounced like the letter x and the following o is like the ou in ought, sought and fought. The final ism sound like the sim in prism. The main stress is on the first syllable. When you exorcise a ghost or an evil sprit, what you do is to drive it away by making use of magic or prayers. This act of driving away an evil spirit is called Exorcism.
• The witch doctor tried to exorcise the evil spirit form the library.