Sexual immorality is any act of sex outside of marriage, and sometimes includes sexual acts even between spouses. Sexual immorality includes fornication, adultery, homosexuality, promiscuity, incest, pedophilia, bestiality, and any form of lust. The term is understood universally across cultures, though the norms slightly change (some cultures, such as the Greeks allowed pedophilia, and late 20th century godless culture allows homosexuality and fornication).
The Greek word πορνεία (porneía), often used in the New Testament, is typically translated "fornication," although the word broadly refers to any form of illicit sexual intercourse, including remarriage after an un-Biblical divorce and those sexual acts that were prohibited under the Mosaic law. The word πορνεία is used in Acts 15:20 and Acts 15:28-29 to describe activities that are forbidden to Gentile Christians. The word can also figuratively mean idolatry.