First of all, the term can be used in several different ways. The term "Gospel Music" is a certain type of Christian music, and the term "The Four Gospels" is a convenient way to refer to the books in the Bible that focus on Jesus' life. What I want to focus on here is how it is defined when used in the last three examples above. What does it mean when a pastor or the apostle Paul refers to "the Gospel," or someone tells you that you should "share the Gospel" with people who don't believe in Jesus.
I believe it can be boiled down to this:
The Gospel is the teaching that every person is under the condemnation of God because of our sins, but that any person can be saved from God's wrath by believing that Jesus died for our sins on the cross and was raised from the dead three days later, by turning away from sin, and by submitting your life to him as your Master.
My basis for this claim comes from two passages in the New Testament:
Now I would remind you, brothers,of the gospel I preached to you, which you received, in which you stand, and by which you are being saved, if you hold fast to the word I preached to you—unless you believed in vain. For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures. (1 Corinthians 15:1-4, ESV)
Because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. (Romans 8:9-10, ESV)In the first passage we see the heart of the Gospel spelled out: that Jesus died on the cross for our sins, was then buried in a tomb, but then was raised back to life on the third day after his death. Believing these historical facts is foundational to being saved from God's wrath. However, merely believing is these facts is not enough, as James points out: "Even the demons believe--and shudder" (James 2:19, ESV).
So in the second passage, another element is added where we are told "confess with your mouth that Jesus is Lord." In other words, we need to acknowledge that Jesus is God and submit our lives to him as our Master. Part of that process involves turning away from sin, which is another cruical element of the Gospel (see Matt. 4:17; Acts 2:38; 3:19; and 2 Pet. 3:9). If you acknowledge that Jesus is your Master, then you will start obeying His commands and turn away from sin.
So the next time you run across the term "Gospel," this is what it means.
For these insights, I am indebted to my mentor and professor C.W. Smith at The Master's College, who is now in the presence of God because he believed in the Gospel and confessed with His mouth that Jesus is Lord.