According to Webster's Dictionary, Substance Abuse is defined as:

Main Entry: substance abuse
Function: noun
Date: 1982
: excessive use of a drug (as alcohol, narcotics, or cocaine) : use of a drug without medical justification

Basically, it is the abuse or misuse of alcohol and illegal drugs in addition to prescription drugs or any other drug that affects the central nervous system and/or the brain

This use usually wreaks havoc on the user and everyone involved with them. The only way to help the abuser is to remove the drug from the system and with behavior modification, therapy, and a tremendous support system.