As used in sections 53.250 to 53.265, unless the context clearly indicated otherwise, the following words mean:
(1) "Assessor or assessing officer", county assessor of all second, third, and fourth class counties, and all first class counties without a charter form of government and the assessing officer of the city of St. Louis;
(2) "Assessor-elect", a person who has been elected or appointed to the office of county assessor in any second, third, or fourth class county or any first class county without a charter form of government, or as the assessing officer of the City of St. Louis, but who has not yet begun his term of office;
(3) "Commission", state tax commission;
(4) "Course of study", course or courses approved by the state tax commission.