Even in Old Testament times, mankind was given ten commandments, reflecting the basic principles of man's relationship to God and neighbors. The list of Sinai legislation contains the commandment that a person should not commit the sin of adultery.
In modern Christian moral teaching, in addition to the sin of adultery, one can often hear about the so-called fornication. It is worth noting that all these manifestations of human sinfulness, according to the teachings of the Orthodox Church, relate precisely to the commandment "do not commit adultery." Let's try to figure out what is the difference between adultery and fornication.
Usually, adultery is understood as adultery. Adultery is the entry of a person into intimacy with someone other than his legal spouse. It is worth noting that "legal" in this context means an official marriage registered at the registry office.
Fornication is called any entry into intimate intercourse outside the marriage bond. That is, sex outside of marriage is usually called that way. To the extent of this, one can often hear about the negative attitude of the Church towards the so-called civil cohabitation. However, in this context, various questions may arise. For example, is it worth confessing fornication to those who for a long time lived an intimate life with a person before legal marriage. At the same time, there was sex with only one partner, with whom the marriage took place later. Some argue categorically, calling such cohabitation before marriage fornication, others are more condescending to human weakness, but still advise to designate such cohabitation in confession.
Thus, it turns out that adultery is adultery, and fornication is entering into intimacy outside of marriage (especially in the case when partners change repeatedly). At the same time, it is necessary to understand that all these manifestations of sinfulness are equally suitable under the prohibition of the Old Testament commandment - "do not commit adultery."
You can also give a conditional synonym for these sins - lust. It should be noted that the violation of the commandment "do not commit adultery" in the Christian tradition is attributed to mortal sins.