Is there a difference between fornication and adultery?

from the old testament

• In the King James and New King James Versions of the Bible, fornication is a word used in relation to sexual sins and idolatry. For example, in 2 Chronicles 21, the fornication or sexual immorality of the people included involvement in idolatry with Baal.
• In Ezekiel 16, fornication was repeatedly used to describe the idolatry of God's people.
• Adultery is specifically listed as prohibited in one of the Ten Commandments (Exodus 20:14). This made it a practice especially condemned in Jewish culture. Most notable was the affair of King David with Bathsheba that caused much trouble (2 Samuel 11—12). Yet even in this case, David later repented and received forgiveness from God, despite the consequences (Psalm 51).

from the new testament

• In the New Testament, fornication is often the translation of the Greek word porneia, which includes sexual immorality of many kinds, both adultery and other sexual sins (Matthew 5:32; 1 Corinthians 6:18).
• In Matthew 5:32 and 19:9 Jesus notes "sexual immorality" (porneia) as a situation in which God permits divorce. Those who divorce and remarry outside of this are said to commit “adultery” (from the Greek moicheuō). The only other reason for divorce specifically mentioned in the Bible is abandonment by an unbelieving spouse (1 Corinthians 7:15).
• In the Bible, adultery is used in the normal sense as sexual relations outside of a person's marriage. While the Bible also uses the word figuratively to speak of the "spiritual adultery" of God's people, that is the case only once in the New Testament (Revelation 2:22).
• James 2:11 notes both adultery and murder as sin before God.
• Yet adultery and fornication are also forgivable for those who repent. First Corinthians 6:9–11 says, “Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God. And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.” The Corinthians “were” the types of people characterized by these sins, but they were saved. Sexual immorality—including fornication and adultery—are forgiven in Christ. Those who have sinned sexually can be saved and transformed. Those who are saved and fall into these sins need not fear loss of salvation (Ephesians 1:3–14). They can experience cleansing and restoration in Christ (1 John 1:8—2:6).

implications for today

God calls us to live pure lives, honoring Him through our bodies and relationships. We are all called to sexual purity and to set our hearts on God’s desires rather than worldly passions (1 Thessalonians 4:3–4). For those who are single, this means abstaining from sexual activity. For those who are married, God calls us to faithfulness, upholding the sacredness of the marriage covenant and protecting it from the destruction that comes from adultery (Hebrews 13:4). Whether single or married, we must all guard our thoughts and our hearts, knowing that sexual sin is not merely a matter of intercourse but also of the mind and heart.

Both fornication and adultery are sins because they distort God’s design and go against His good ways. Marriage is an image of Christ and the church (Ephesians 5:31–32) and a metaphor God often used in the Old Testament to describe His relationship with His people. Sex is designed for one man and one woman in the covenant of marriage (Genesis 2:24). Sex outside of those boundaries distorts the image of God’s relationship with His people, thereby giving us a false view of Him. Adultery and fornication also distort His design for marital intimacy and undermine the trust and love He intends for human relationships. When we choose purity, we reflect God’s holiness and experience the freedom and wholeness that come from living according to His perfect will. When we have distorted God’s good design and broken trust by engaging in sexual sin, God has grace. We can receive His forgiveness and restoration, and move forward in purity and freedom.