A Greek word meaning "Good News" it is often used in the New Testament of the Christian Bible to refer to news of the savior, even Jesus Christ.
This word has been the source of some confusion to date, particularly from Paul's use of it in Galatians 1:6-8 KJV -
"I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: Which is not another; but there be some that trouble you, and would pervert the gospel of Christ. But though we or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed."
Jesus here is not talking about the gospels of Matthew, Mark, Luke and John. These had not been written at the time of Galatians. He is condemning gospels which are not of Jesus Christ. If it is about Jesus Christ and how he is the savior, he's not condemning it.
Don't preach other false doctrines (gospels) which say cows or aliens will redeem us, lest you be accursed.