Fornication is voluntary sexual intercourse between two unmarried people, more often called adultery. Fornication (and adultery) is prohibited in the Bible.
The King James Version of the Bible uses the word fornication, whereas most other versions use the word adultery instead.
Key verse about fornication: Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God (Galatians 5:19-21, NKJV).
[Read other verses in the Bible about fornication in the KJV.]
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 1,000-page website ABibleADay.com to encourage people to explore the Bible. His main blog and many books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.