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. "Substitute teacher" (usually abbreviated as "sub") is the most commonly used phrase in the United States, Canada (except Ontario), India and Ireland, while supply teacher is the most commonly used term in Great Britain and the Canadian province of Ontario. Common synonyms for substitute teacher include relief teacher or casual teacher (used in Australia and New Zealand) and "emergency teacher" (used in some parts of the United States). Other terms, such as "guest teacher", are also used by some schools or districts. Regional variants in terminology are common, such as the use of the term teacher teaching on call (TTOC) in the Canadian province of British Columbia and occasional in Ontario.