Nolo's Plain-English Law Dictionary

Circumstances that increase the seriousness or outrageousness of a given crime, which will increase the wrongdoer's penalty or punishment. For example, the crime of aggravated assault is a physical attack made worse because it is committed with a dangerous weapon, results in severe bodily injury, or is made in conjunction with another serious crime. Aggravated assault is usually considered a felony, punishable by a prison sentence.

See Aggravating Circumstances in Sentencing for more detail.