What is the meaning of the Greek word porneia in the Bible?
The Greek word porneia occurs 25 times in 24 verses in the New Testament. The word appears for the first time in the New Testament in Matthew 5:32.
. . . but I say to you that everyone who divorces his wife, except for the reason of unchastity, makes her commit adultery; and whoever marries a divorced woman commits adultery. Matthew 5:32 (NASB)
The word porneia is translated differently in different Bibles. For example, in the New American Standard Bible (NASB) the word is translated as “unchastity.” The King James Version translates the word as “fornication.” The New International Version (NIV), NET Bible and English Standard Bible (ESV) translate porneia as “immorality.” The New Living Translation (NLT) translates the word as “unfaithful” and, unfortunately, misses the meaning of the word.
The Meaning of Porniea – Greek Lexicons
Now we will discover what the Greek dictionaries say the word means. A Greek dictionary is called a Greek lexicon. We will examine a number of lexicons so that the reader can see they are in agreement even though the wording may be slightly different. But first, we will start with an overview.
The original meaning of the Greek word porneia is to prostitute or to sell. However by the time of the New Testament, porneia had a very broad meaning that included sexual behavior such as prostitution, extramarital sexual intercourse or adultery, paedophilia, promiscuity, homosexuality, lesbianism, incest, premarital sex and bestiality. The Exegetical Dictionary of the New Testament states,
PORNEIA means “prostitution, unchastity, fornication,” and is used “of every kind of unlawful sexual intercourse” (BAGD s.v.). . . . Since in Rom. 1:26f. Paul clearly alludes to homosexuality as sexual immorality, PORNEIA can also refer to homosexuality as sexual immorality . . . 
Kittel, Bromiley and Friedrich provides a very complete meaning of porneia stating that its meaning includes “adultery, fornication, licentiousness, and homosexuality.”[2, 3] Harper’s Bible Dictionary states that porneia also includes “bestiality.”
Colin Brown states that . . .
[Porneia] can describe various extra-martial sexual modes of behavior insofar as they deviate from accepted social and religious norms (e.g. homosexuality, promiscuity, paedophilia, and especially prostitution).
Colin Brown also tell us that porneia occurs in the “Testament of Benjamin.” The word is used to refer to homosexual behavior. In the following quote from the “Testament of Benjamin” porneia is translated as fornication.
And believe that there will be also evildoings among you, from the words of Enoch the righteous: that ye shall commit fornication with the fornication of Sodom . . . 
Verein D. Verbrugge in the New International Dictionary of New Testament Theology writes . . .
Rab. Jud. frowned on any kind of prostitution or extramarital sexual intercourse. Incest and all kinds of unnatural sexual intercourse were viewed as porneia. 
How Is Porneia Used In the New Testament
1 Corinthians 7:2 makes it clear that porneia refers to sexual intercourse and sexual activity.
But because of immoralities, each man should have relations with his own wife and each woman with her own husband. 1 Corinthians 7:2 (NASB)
The word “immoralities” in this verse is translated from porneia. This helps us understand that porneia refers to some form of sexual activity, including sexual intercourse. The following chart shows us how porneia is used in the New Testament and helps us understand the breadth of the meaning of porneia.
The Greek word porneia includes all illicit sexual intercourse or sexual activity. The word can refer to prostitution, extramarital sexual intercourse, paedophilia, promiscuity, homosexuality, lesbianism, incest, premarital sex and bestiality.
1. Balz and Schneider. Exegetical Dictionary of the New Testament. Eerdmans Publishing Com. 1993. pp. 137-139.
2. Kittel. Theological Dictionary of the new Testament. Eerdmans. 1968. vol. vi., p. 581-595.
3. Bromiley, Geoffrey W. Theological Dictionary of the New Testament. Eerdmans Publishing Com. 1985. VI, pp. 918-921.
4. Achtemeier, Paul J. “Fornication.” Harper’s Bible Dictionary. Harper & Row & Society of Biblical Literature. 1985, p. 319.
5. Colin Brown. Dictionary of New Testament Theology. Regency Publishers. 1975. vol. 1., p. 497.
6. Ibid. p. 499.
7. Verlyn D. Verbrugge. New International Dictionary of New Testament Theology. Zondervan. 2000. pp 486.