master

a combatant who is able to defeat rivals

an officer who is licensed to command a merchant ship

an artist of consummate skill

“a master of the violin”

have a firm understanding or knowledge of; be on top of

most important element

“the master bedroom”

directs the work of others

someone who holds a master's degree from academic institution

get on top of; deal with successfully

an authority qualified to teach apprentices

key that secures entrance everywhere

be or become completely proficient or skilled in

“She mastered Japanese in less than two years”

an original creation (i.e., an audio recording) from which copies can be made