"A substitute teacher is a person who teaches a school class when the regular teacher is unavailable; e.g., because of illness, personal leave, or other reasons. While "substitute teacher" is the generally used phrase in the United States, Canada, Ireland and often Australia, supply teacher is the more common term in the United Kingdom (though the term is also used in Canada), Relief teacher is also used in Australia and New Zealand. An "emergency teacher" or a "sub" is another name for a substitute teacher in the US and Australia. There are also various other terms which may be used in a particular region (such as teacher on call T.O.C. in the Canadian province of British Columbia)."
http://en.wikipedia.org/wiki/Substitute_teacher