'Slave' in the Bible

Were you called while a slave? Do not worry about it; but if you are able also to become free, rather do that.

For he who was called in the Lord while a slave, is the Lord’s freedman; likewise he who was called while free, is Christ’s slave.

For though I am free from all men, I have made myself a slave to all, so that I may win more.

but I discipline my body and make it my slave, so that, after I have preached to others, I myself will not be disqualified.