Antiviral: An agent that kills a virus or that suppresses its ability to replicate and, hence, inhibits its capability to multiply and reproduce.
For example, amantadine (Symmetrel) is a synthetic antiviral. It acts by inhibiting the multiplication of the influenza A virus. It was used to lessen the severity of the disease, particularly in individuals at high-risk such as those who are immunosuppressed or in a nursing home. Amantadine has been replaced by safer medicines, oseltamivir (Tamiflu) and zanamivir (Relenza) which have fewer side effects.
The development of antivirals has lagged far behind that of antibiotics. A virus is just genetic material, DNA or RNA, perhaps with a few enzymes, wrapped in a protein coat. A viral is technically not alive which makes it hard to kill. Further, viruses replicate (make copies of themselves) by hijacking the machinery of the cell they infect, so it is difficulty to kill the virus without killing the cell. Some viruses can also remain dormant in the body without replicating, thereby avoiding drugs that inhibit replication.
The antivirals that have been developed are generally less effective than one would like. Viruses can replicate rapidly and, in many cases sloppily, giving rise to mutations that make them resistant to drugs. And for fast-moving viral infections like flu or a cold, a drug must be very powerful to make a difference before the disease runs its natural course.