First, notice that the question: "What is a Christian?" is quite different from the question: "What is a born again child of God?"
In this text, we are only attempting to answer the question ... "What is a Christian?"
According to the Bible, (Acts 11:19-26) it is recorded that the disciples of Jesus Christ were first called Christians in the community of Antioch.
Acts 11:26 NKJV says that: "it was that for a whole year they assembled with the church and taught a great many people. And the disciples were first called Christians in Antioch."
The three key words here are "church, disciple, and Christian."
Greek lookup on BibleHub:
church: an assembly, a (religious) congregation
disciple: a learner, pupil
Christian: follower of Christ.
The above definition of the word Christian in the context of Acts 11 should suffice as the answer.
However, in dealing with various individuals, it is good to remember that others do not necessarily understand or agree with the above definitions.
It is also good to realize that the understanding of the English word "Christian" has changed a bit over the years. In fact, the English language itself hasn't been around for a very long time. Grace is necessary to facilitate a productive conversation with so many variables.
So, here goes, with regard to the English definition...
The 1828 Webster's dictionary has a clearly understandable definition of the word "Christian." Depending on the source, the more modern definitions can be a bit confusing in my opinion. With that said, here's the definition from the 1828 Webster's dictionary...
--- BEGIN DEFINITION ---
1. A believer in the religion of Christ.
2. A professor of his belief in the religion of Christ.
3. A real disciple of Christ; one who believes in the truth of the christian religion, and studies to follow the example, and obey the precepts, of Christ; a believer in Christ who is characterized by real piety.
4. In a general sense, the word Christians includes all who are born in a christian country or of christian parents.
CHRISTIAN, adjective [See the Noun.]
1. Pertaining to Christ, taught by him, or received from him; as the christian religion; christian doctrines.
2. Professing the religion of Christ; as a christian friend.
3. Belonging to the religion of Christ; relating to Christ, or to his doctrines, precepts and example; as christian profession and practice.
4. Pertaining to the church; ecclesiastical; as courts christian
CHRISTIAN, verb transitive To baptize.
--- END DEFINITION ---
Conclusions: According to the Bible, a Christian is a "follower of Christ." According to the English language, a Christian can be many different things. However, since the word of God has been around a lot longer than the English language, it is best to stick with the Bible definition.
A Christian is a "follower of Christ."
God bless you!