1 : a male ruler of a country who usually inherits his position and rules for life [count] ▪ dethrone a king ▪ He pledged his loyalty to the king and queen. ▪ the reign of King James [noncount] ▪ He was crowned king. ▪ He became king at a young age.
2 a [count] : a boy or man who is highly respected and very successful or popular ▪ an oil king ▪ He's widely regarded as the king of soul music. b [count] : a boy or man who is awarded the highest honor for an event or contest ▪ He was voted king of the prom. ▪ the homecoming king c : something that is very powerful or that is considered better than all others [count] ▪ The lion is known as the king of the jungle. [noncount] ▪ It was a time when comedy was king. [=when comedy was the most important or popular form of entertainment]
3 [count] : the most important piece in the game of chess ▪ She won the game by checkmating his king. —see picture at chess
4 [count] : a playing card that has a picture of a king and that is worth more than a queen ▪ the king of hearts/spades/clubs/diamonds —see picture at playing card