This is the definition I found on several different dictionaries online, I suppose it's what defines the most used meanings of the word "slave": "a human being who is owned as property by, and is absolutely subject to the will of, another; bondservant divested of all freedom and personal rights
a person who is completely dominated by some influence, habit, person, etc. a slave to fashion
a person who slaves; drudge
slave ant
a device actuated or controlled by another, similar device"

You'll have to forgive me if my english is a bit rusty...

Personally I think that the term "slave" is beeing used too frequently in the world of BDSM...