For the Old Testament, a modern tool will translate the hebrew ערוה as “unchastity”, “nakedness” or “incest”.
This term is translated in modern versions as “sexual immorality”, but that expression is very vague. In old translations that expression was translated as “fornication”.
What's the real meaning from the Hebrew view at the time?
In the New Testament, the expression “sexual immorality” cames from the Greek πορνεια, and it's meaning it's explained in this question.