What is Fornication?
The Hebrew and Greek words are listed below. From the words you can see that prostitution is the primary meaning, but adultery, idolatry and incest can be included. Idolatry is better understood if one knows the scriptural story of God's marriage divorce and promised remarriage to true Israel. Prostitution seems to be the primary meaning. I have seen people add other things they think are sexual sins, but as you can see nothing else is mentioned in Strong's Concordance. Fornication is a more general term for intercourse when it is sin.
In relation to the term one may consider the following.
The above are the primary sexual sins found in scripture. These protect the lineage of the man and his wife or future wife. The primary sex of scripture is penile-vaginal intercourse. Notice that the marital status of the man is never mentioned in scripture. Whereas if the sin of adultery is mentioned then you know the woman was married. A married woman must be involved for the sin to be adultery.
The other sexual sins include anal intercourse and incest. Anal intercourse is never permitted between a man and a woman, Romans 1:26, nor between a man and a man, Romans 1:27. Incest is intercourse between close relatives as listed in Leviticus 18. Here is another example of the error of ignoring the old testament. If one ignores the old testament then the only prohibition to a close relative is found in 1 Corinthians 5:1. Bestiality is also only found in the old testament. God's morals do not change.
One thing I do try to point out is that we have to separate the bad attitude of some of the early church fathers toward sex. If they had had their way none of us would be alive today. They preferred that one never have even marital intercourse or any sexual enjoyment. Their heresy was from Gnosticism and Asceticism. To varying degrees those philosophies teach that anything physical was evil and any enjoyment of the physical was a great evil. They are anti-Christ if they refuse to believe that Christ came in the flesh.
It is probably necessary to mention that masturbation is never once mentioned in scripture. It is not a sin, but a gift from God and can be necessary as one ages. The one example that is used has to do with Onan found in Genesis 38:9. That is a whole story in itself since Judah sinned by marrying a Canaanite, one of the forbidden lineages for Israel to marry. Canaanites, like Moab and Ammon were children of incest. Onan was supposed to fulfill a public promise to raise up children for his dead brother. God's providence had the line of Judah continue through Tamar.
In the old testament I found four related Hebrew words. The first appears to be the root word for the others.
I found five Greek words of interest.
Likely there is more to add to the word list or be more specific as time goes.
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Words Mean Things!
What the Bible says on Divorce and Putting Away. (An in depth look.)
What Constitutes Adultery?
What is Fornication?
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Let each have her own husband!
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The Marriage, Divorce and Remarriage of God to Israel
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