Originally denoting Saturday, the seventh day of the week, or, more precisely, the time period from Friday sunset to Saturday sunset, the term 'sabbath' can now mean one of several things, depending on the context and the speaker:

The special significance of the seventh day of the week, called the Sabbath, begins with God's creation of man on the earth. According to the book of Genesis, the first book of the Bible, God created man on the sixth day; and on the seventh day "he rested from all his labors," and therefore sanctified (made holy) the seventh day (Genesis 2:2-3 , Exodus 20:11).

In the New Testament, Jesus declared that he was the Lord of the Sabbath (Matthew 12:8 , Mark 2:28), and that it was made for man's good (Mark 2:27). Jesus, in his teachings, rebuked the Jews, and taught that it was right to do good on the Sabbath (Mark 3:4 , Luke 6:9). The Sabbath continued to be a time of communal gathering for Christians (Hebrew 10:25), as well as learning the will of God (Acts 15:21). Christians continued to observe the seventh day as holy for centuries after the crucifixion of Jesus. First day observance (also called the Lord's Day, on Sunday) became traditional in the Catholic (Roman) and then later the Orthodox (Greek) churches. Though greatly reduced in number, some Christians continued to observe the seventh day Sabbath, even up to this day.

Sabbath - Christianity Knowledge Base

Sabbath Truth Website

Wikipedia - Sabbath  |  Shabbat

Wikia - Sabbath

Theopedia - Sabbath

Merriam-Webster Dictionary - Sabbath

Answers.com - Sabbath

100 Questions - Sabbath (PDF)