(A word from the Latin, 'evangelium', meaning, "a good or joyful message".)
is,
The story of the life of Jesus Christ; including the revelation of God's grace to fallen man and His plan of salvation through His Son (Jesus Christ), as revealed by Jesus and His apostles.

...For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
Romans 1:16