Marriage is honorable in all, and the bed undefiled:
but whoremongers (fornicators) and adulterers God will judge.
The study uses a systematic method of word study to define the word "fornication" by only comparing the Scriptures by the Scriptures. In this method, we start with a clear New Testament teaching, find the Greek word, then find a verse that will send us back to the Old Testament, for which we can then do a systematic study of the Old Testament.
Here we start at Hebrews 13:4 and from our concordance find that the Greek word for whoremongers/fornicators is "pornos" (Strong's 4205). By looking up the word "pornos" we can find that there are three Greek words that are very similar: "porne" (Strong's 4204) which is translated as "harlot" or "whore" and "porneuo" which is translated as "commit fornication". Of all these, we desire to find a verse that will send us back to Old Testament Hebrew. In Hebrews, we can find a verse that relates back to the Old Testament:
By faith the harlot Rahab perished not with them that believed not,
when she had received the spies with peace.
This verse refers back to the book of Joshua in the Old Testament. Specifically Joshua 6:17 says:
And the city shall be accursed, [even] it, and all that [are] therein, to the LORD: only Rahab the harlot shall live, she and all that [are] with her in the house, because she hid the messengers that we sent.
From this verse, we can find that the Hebrew word for "harlot" is "zanah".
Systematic word study of "zanah" (02181)
Now we look specifically for the word "zanah" for Scriptures that have some sort of context.
Context verse #1: Dinah
We find that the first place it is used is in Genesis 34:31. However for context, we will list Genesis 34:1-9:
And Dinah the daughter of Leah, which she bare unto Jacob, went out to see the daughters of the land. And when Shechem the son of Hamor the Hivite, prince of the country, saw her, he took her, and lay with her, and defiled her. And his soul clave unto Dinah the daughter of Jacob, and he loved the damsel, and spake kindly unto the damsel. And Shechem spake unto his father Hamor, saying, Get me this damsel to wife.
Then in verse 31, it is written:
And they said, Should he deal with our sister as with an harlot?
Context verse #2: Tamar
The Hebrew word "zanah" is also translated in Genesis 38:15 as harlot:
When Judah saw her (Tamar), he thought her to be a harlot;
because she had covered her face.
Context verse #3: Leviticus 19:29
Do not prostitute thy daughter, to cause her to be a whore;
lest the land fall to whoredom,
and the land become full of wickedness.
Context verse #4: Deuteronomy 23:17-18
because she hath wrought folly in Israel,
to play the whore in her father's house:
so shalt thou put evil away from among you.
Context verse #4: Deuteronomy 23:17-18
There shall be no whore of the daughters of Israel,
nor a sodomite of the sons of Israel.
Thou shalt not bring the hire of a whore, or the price of a dog, into the house of the LORD thy God for any vow:
for even both these are abomination unto the LORD thy God.
Here a whore is also one who prostitutes or sells herself for hire.
Context verse #6: Ezekiel 16
Ezekiel 16 is filled with different variations of the word harlot, whore, and fornication. And thus gives a good context for what a harlot is: a beautiful woman that rose up and went after everyone she could.
v4. ...in the day thou wast born thy navel was not cut, neither wast thou washed in water to supple thee...
Comparing with other sources
Once we have studied by the Scriptures alone what "fornication" means, then it is helpful to compare with the conclusions to which others have come. For example, BlueLetterBible.org gives the following definition:
What is fornication?
Fornication comes from the Greek word porneia (4202) and means (1):
1) illicit sexual intercourse
1a) adultery, fornication, homosexuality, lesbianism, intercourse with animals, etc.
1b) sexual intercourse with close relatives; Leviticus 18
1c) sexual intercourse with a divorced man or woman; Mark 10:11,12
2) metaph. the worship of idols
2a) of the defilement of idolatry, as incurred by eating the sacrifices offered to idols
There are 26 instances of the word porneia in Mat 5:32, 5:19, 19:9, Mat 7:21, John 8:41, Acts 15:20, 15:29, 21:25, Romans 1:29, 1Cor 5:1, 6:13, 6:18, 7:2, 12:21, Gal 5:19, Eph 5:3, Col 3:5, 1Th 4:3, Rev 2:21, 9:21, 14:8, 17:2, 17:4, 18:3, 19:2.
Fornication is the wreckless setting aside of what God has clearly stated, entering into a relationship which was designed only under a marriage covenant.