Eternal life is the life everlasting in the presence of God. "This is eternal life, that they may know you the only true God, and Jesus Christ, whom you have sent." (John 17:3).
There are two senses in which this is used. First, Christians possesses eternal life (1 John 5:13), yet they are not in Heaven or in the immediate presence of God. Though man is still in their mortal bodies and he still sins, however, by faith man is saved (Rom. 4:5, Eph. 2:8-9) and possess eternal life as a free gift from God (Rom. 6:23). Second, eternal life will reach its final state at the resurrection of the believers when Christ returns to earth to claim His church. It is then that eternal life will begin in its complete manifestation--especially seen in that man will no longer sin.