Abuse in the pharmaceutical sense means use improperly. Source DEA Drugs of Abuse]
The use of illegal drugs is abuse by this definition. The reason being that there is no intended proper use for those drugs.
Take the case of heroin. Heroin (diacetylmorphine) has no therapeutic use that is recognized by the DEA. Its only recognized use (by US law) is to get high. Hence, its use is always considered to be abuse. (Note it is rarely used for extreme pain management in the UK and some other countries, but its use is highly regulated.)
Marijuana and cocaine, by contrast have recognized therapeutic uses. Marijuana can be used for intractable nausea, glaucoma, extreme cases of anorexia, and other recognized uses. Cocaine is a both a local anesthetic and vasoconstriction agent which is used in ENT and ophthalmologic surgeries. When used in the properly prescribed conditions, the use of these agents is not abuse. But, when used for non-prescription uses (recreational, etc.) it constitutes abuse.
People frequently mistake abuse for addiction. That is a separate concept. Addiction is the excessive, habitual use, and implies a psychosocial dependency on the drug that outstrips its usefulness therapeutically. Contrast this with dependance, which represents a physical need for a drug without the underlying psychosocial components or excesses.
Source: I am a physician who practices Anesthesiology. I hold a DEA license to prescribe controlled substances. And, I have training in the administration of controlled substances, and diversion prevention.