Real Dictionary

king
A king is a man who is the most important member of the royal family of his country, and who is considered to be the Head of State of that country.
countable noun: A king is a playing card with a picture of a king on it.
countable noun: If you describe a man as the king of something, you mean that he is the most important person doing that thing or he is the best at doing it.

King in American English
(William Lyon) Mackenzie1874-1950; Cdn. statesman: prime minister (1921-26; 1926-30; 1935-48)
a male ruler of a nation or state usually called a kingdom; male sovereign, limited or absolute; monarch
intransitive verb: to reign as king
noun: a male sovereign or monarch; a man who holds by life tenure, and usually by hereditary right, the chief authority over a country and people
transitive verb: to make a king of; cause to be or become a king; crown

king in British English
noun: a male sovereign prince who is the official ruler of an independent state; monarch
noun: B.B., real name Riley B. King.1925–2015, US blues singer and guitarist