Engaging in consensual sex, between two persons who are both unmarried.
When two persons engage in a sexual relations, both consenting, and both are not attached, then it is fornication.
Both have to be unmarried.
Sex has to be consensual.
Originates from the Latin Word fornix, which means archway, vault.
It was widely used in reference to those who used prostitutes, in ancient times, as they were found under the arches in early Rome.
Due to the euphemism used in Ancient Rome, fornication has become a rather derogatory term, for sex.
However, it merely denotes sex between unmarried partners, which is more of a moral issue, than anything else.
Most societies base their laws, code, on religious morals.
The New Testament has often translated the word 'porneia' as meaning fornication, when in reality it mean 'sexual immorality or sexual perversion'.
This has led to further misuse of the word, and its connotation in society.
Practice (Associated Acts):
Most common law countries were never really interested in prosecuting citizens for sexual deviations, within their own domain (home).
Sodomy was an exception to that practice.
Even Incest was barely legislated against, unlike Sodomy, providing the two involved parties were of legal age.
Mostly in Islamic countries, fornication is a criminal offence.
Modern terms used, to denote Fornication, was Premarital Sex, again something frowned upon by most mainstream Religious Orders.
Early attempt by many Southern US States, to make Fornication illegal, was eventually struck down.