A more thorough explanation:

Definition: A deviation or divergence from a standard rule, regulation, measurement, or course of conduct that is impermissible.

Example: In the federal sentencing guidelines, a forbidden departure is an impermissible deviation from the guidelines based on race, sex, national origin, creed, religion, or socioeconomic status.

This means that a judge cannot give a harsher or more lenient sentence based on a person's race, gender, or other personal characteristics. The sentence must be based solely on the crime committed and the guidelines set forth by the law.