Jesus makes it clear that adultery occurs in remarriage. And if a woman shall put away her husband, and be married to another, she committeth adultery. Jesus added to this and stated that a man can commit the sin of adultery in the fantasy of his mind without the physical act taking place. Jesus said that for a man to desire in his mind to have sexual relations with another woman, that man commits adultery in his heart. According to the definition for the Greek word for adultery that Jesus used, the act or action that is adultery in remarriage is voluntary sexual intercourse with a person whose first husband or wife is still alive.
Divorce is a serious act of defilement against the spirit and every other aspect of the marriage.
But nowhere in the Bible does God say that divorce is adultery. There is no definition in any dictionary or Greek lexicon that says that the action of defiling the marriage is adultery. Adultery is voluntary sexual intercourse, either mentally or physically, where at least one of those involved is married to someone else. Defilement of a marriage is a consequence of adultery. But neither defiling a marriage nor divorce is adultery.