The titles "Old" and "New Testament" were used by St. Paul (2 Corinthians, 2:14). The term "testament," as applied to the two parts of the Bible, means: a covenant, agreement, pact. In the language of the Bible it denotes the agreement or pact between God and man: Man agreed to do certain things and God, in return, promised certain blessings. The Old Testament contains a record of the pact between God and Abraham and between God and Moses. The New Testament is an account of the pact between God and His creatures. Both the old and the new covenants were sealed by blood: The pact between God and Abraham was sealed by the circumcision (Genesis 17); the pact between God and the Jewish people, by the sprinkling of the people with the blood of animal victims (Exodus 24:7, 8); the pact between God and men, by Christ's own blood (Matthew 26:28; 1 Corinthians 11:25). Besides denoting the Jewish and Christian religions, the terms "Old" and "New" Testaments also designate the sacred books of each.
Material taken from http://www.cathtruth.com/catholicbible/bookbook.htm