I have not been able to find any reference to define the use of the word adultery from a Biblical perspective except that it was and is a sin of lust between a married person and another person.
Does this mean that lustful thoughts for a single person are not sin? I would beg to differ. Lustful thoughts are covetous thoughts and coveting is sin. Scripture speaks much about the sins of adultery, lust, impurity, sexual immorality, sensuality, homosexuality, incest, bestiality, and other unnatural desires. Just to name a few....
Matthew 5:28 (lust, adultery)
Ephesians 5:3 (sexual immorality, impurity, covetousness)
1 Thessalonians 4:3-5 (sexual immorality, passions of lust)
Galatians 5:19 (works of the flesh: sexual immorality, impurity, sensuality)
Colossians 3:5 (sexual immorality, impurity, passion, evil desire, and covetousness)
1 Corinthians 6:9-11 (sexual immorality, adultery, homosexual activity)
1 Peter 2:11 (passions of the flesh)
Leviticus 18:6-30 (incest, homosexual activity, bestiality)
Jude 1:7 (sexual immorality and unnatural desires like Sodom & Gomorrah)