Checkpoint wrote:
It seems to me it is seldom if ever specifically defined in scripture, although the term is used more than a few times.
Does it, for example, include polygamy?
How about sex before marriage?
Or homosexuality?
Adultery is a sin, but is it also fornication?
The term fornication comes from the Latin fornix, referring to the arch, the arch of the brothel. So, properly, it means prostitution.
The Greek term porneia comes from the word sell, and refers to selling sexual services, i.e. prostitution.
The Hebrew zanah likewise means prostitution.
So it does NOT generally include polygamy, premarital sex, homosexuality nor adultery.
However the terms are used in extended senses. But in what direction is the extension, and how far does it go? Or are there multiple extensions?
Those are complex questions and the answer depends partly on the time and cultural/linguistic background of the speaker or writer.
Tamar, the widow, was found to be pregnant when she had no husband and had been denied access to her husband's brothers. Although she got pregnant by pretending to be a prostitute, she is charged with, in effect, being an unmarried woman who somehow got pregnant to an unknown man. This sin was zanah, meaning prostitution. So, simply getting pregnant as an unmarried woman seemed to be enough. Cash payment and promiscuity don't seem to be required.
The unmarried young woman who was supposed to be a virgin, but who was found not to be on her wedding night, is said to be guilty of zanah also (Deut. 22). Again, cash payment and promiscuity don't seem to be required. It is sufficient that the never married woman who was supposed to be a virgin had lost her virginity to some other man before she had her first husband.
Zanah is never used for the sin of the married woman, other than prostitution itself, i.e. the sexual sin of the married woman, short of prostitution, is NEVER referred to as prostitution. There is, of course, another term for the generic sexual sin of the married woman: adultery.
A married woman could commit adultery AND zanah only by ACTUAL prostitution, as was the case of Israel and Judah, women married to YHWH, who turned to ACTUAL prostitution with the foreign nations and their kings and gods. Another case is following conquest, one's wife could be made a prostitute (e.g. Amos 7:17). Other than this kind of case, a married woman never commits zanah.
Other illegal sexual relationships or marriages are not referred to as zanah. Sexual relationships or marriages with close relatives and those of former spouses are never described as zanah. However, as such relationships are not valid lawful marriages, if the woman concerned is unmarried, it may be described as zanah, i.e. illegal sexual relations with an unmarried woman. For example, if a man 'marries' or has sexual relations with his (late) father's wife, that is not a valid marriage, and the (late) father's wife, is a widow, an unmarried woman, and so her unlawful sexual relations is a species of zanah / porneia (e.g. 1 Cor 5:1). If the father was still alive and if she were still married to him, her sin would not be zanah / porneia, it would be adultery.
This is basically the Hebrew / Jewish use of zanah and porneia. Adultery and homosexual conduct are generally not included. Even male prostitution isn't really included, such being described as dogs rather than zanah.
Adultery, in this context, this early Hebrew / Jewish context, was the unlawful sexual relations of the married woman. Such are unlawful for the reason that they are in breach of the duty of exclusive fidelity to the husband. The husband having sexual relations with an unmarried woman is not adultery. If she were married, it would be adultery (against her husband, not against his wife). Likewise, if the married woman takes another woman as a wife, it is not adultery against his first wife.
As time goes on, and as we move away from Hebrew / Jewish context, things change. Jesus said a man who divorce his wife and took another woman committed adultery AGAINST HER (Mark 10:11). This is an extension of the concept of adultery, applicable under re-instituted and required monogamy in marriage. In the Roman / Greek world, prostitution was equally male prostitution as female prostitution, and adultery was equally a man's unfaithfulness with his wife as a wife's unfaithfulness to her husband. And Greek / Roman usage extended porneia to any illicit sexual activity, rather than limiting it to specifically the sexual sin of the unmarried woman.
However, the vast bulk of the biblical literature is based on Hebrew / Jewish usage. So, in short, when the New Testament uses porneia, it means prostitution, or in an extended sense, the sexual sin of the unmarried woman, and it probably does not extend to adultery, homosexual conduct or even male prostitution.