Definitions don’t change. Opinions (which are largely irrelevant) and consequent interpretation of the meaning of a word change.
At its simplest, ‘adultery’ is defined as voluntary sexual intercourse between a person with someone who is not her/his spouse. Pretty easy conceptually...
By definition, individuals in a married couple scenario who engage in sex with people other than their spouses are adulterers.
In the male case, like it or not, if your wife engages in sex with another man, you are a cuckold, she is a cuckolding her husband.
The abstract concepts stemming from religion, culture, etc. that relate to ‘sin’ or culturally morally objectionable behavior are based in mythology and imagination.