Yes, master is a Scrabble word!

Verb

1. Be or become completely proficient or skilled in

2. Get on top of; deal with successfully

3. Have dominance or the power to defeat over

4. Have a firm understanding or knowledge of; be on top of

Adjective

1. Most important element

Noun

1. An artist of consummate skill

2. A person who has general authority over others

3. A combatant who is able to defeat rivals

4. Directs the work of others

5. Presiding officer of a school

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

7. An officer who is licensed to command a merchant ship

8. Someone who holds a master's degree from academic institution

9. An authority qualified to teach apprentices

10. Key that secures entrance everywhere