Master

Used to translate dominus or kurios (and a synonym of lord in this sense).

But also used to translate magister (but not exactly a synonym of teacher).

Which sense is being used to refer to one's teacher? I would think the second. I believe master in the second second is what translates the degree and title conferred upon those who received the appropriate university education in the middle ages and thus had a license to teach.
One is a teacher of some science or skill or art, having 'mastered' it.

It is not the same as being called a master because one is the master of some person. Today in our egalitarian age this meaning of master is unpleasant. I should look up the meaning of si, as in si fu.

Ckhnat has a post on the use of master or lord to refer to one's husband (after the example of Sarah in the OT. This would make me a bit uncomfortable, and seems to me to be an example of illegitimate or sinful patriarchy. A wife is not a slave or maid of the husband. Still, the Church teaches that is a rightful exercise of authority by the husband over the family (as a social unit).

John Finnis critiques Aquinas' account of the husband-wife relationship in his Aquinas, Moral , Political, and Legal Theory, pages 171-5. What would Finnis make of Fr. Check's The Authority of the Husband According to the Magisteruim?

From Christian Order, Karol Wojtyla and the Patriarchal Hierarchy of the Family by G.C. DILSAVER

Edited, 5 April 2007:
Holy Thursday at St. Columkille was a mixed English-Spanish Mass. During the Mass I was thinking of master again, and was look at the Spanish maestro, which seems to be derived from magister. Though the etymology of master is not as obvious to me, I came home to check and see what info was available online. This is what I found at Online Etymology Dictionary:

master (n.)
O.E. mægester "one having control or authority," from L. magister "chief, head, director, teacher" (cf. O.Fr. maistre, Fr. maître, It. maestro, Ger. Meister), infl. in M.E. by O.Fr. maistre, from L. magister, contrastive adj. from magis (adv.) "more," itself a comp. of magnus "great." Meaning "original of a recording" is from 1904. In academic senses (from M.L. magister) it is attested from 1380s, originally a degree conveying authority to teach in the universities. The verb is attested from c.1225.

I don't think Master should be used as a synonym for Lord (Kurios, Dominus), but if the etymology dictionary is correct, it gained the sense of "one having control or authority" rather early.