In modern Christian moral teaching except for the sin of adultery, can often hear about the so-called fornication. It is worth noting that all these manifestations of human depravity, the doctrine of the Orthodox Church, refer specifically to the commandment "thou shalt not commit adultery". Try to understand what is the difference between adultery and fornication.
Usually understand adultery adultery. Adultery - the entry of the person into intimacy with anyone other than their lawful spouse. It should be noted that "lawful" in this context refers to an official marriage registered in the registry office.
Fornication also referred to as any entrance in sexual intercourse outside the bonds of marriage. That is, sex outside of marriage is usually called in this way. To that extent, we often hear about the negative attitude of the Church to the so-called civil cohabitation. However, in this context, there can be various issues. For example, whether to confess fornication to those who lived a long time sex life with someone before legal marriage. At the same time, you've had sex with only one partner, which happened later marriage. Some argue adamantly, calling such cohabitation before marriage fornication, others more down to human weaknesses, but still suggest to designate such cohabitation in the confessional.
Thus, it appears that the adultery is adultery, and fornication - entry into intimacy outside of marriage (especially in the case when partners change repeatedly). Thus, it should be understood that all these manifestations of sinfulness equally suitable under the ban of the old Testament commandments, "thou shalt not commit adultery".
You can also give a conditional synonym of these sins - lust. It is worth noting that the violation of the commandment "thou shalt not commit adultery" in the Christian tradition refers to mortal sin.