Noun, pl. viruses
1. A small infectious agent that can replicate only inside the living cells of an organism. Viruses can infect all types of organisms, from animals and plants to bacteria and archaea. (Wikipedia.org)
2. An infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host. (Google Dictionary)
3. An infection or disease caused by such an agent. (Google Dictionary)
4. A harmful or corrupting influence. (Google Dictionary)
5. A piece of code that is capable of copying itself and typically has a detrimental effect, such as corrupting the system or destroying data. (Google Dictionary)
Word origin: Latin virus (“venom or poison”).