Final answer:
A single virus particle outside of a host cell is called a virion, which consists of a nucleic acid core surrounded by a protein coat known as a capsid.
Step-by-step explanation:
A single virus particle is known as a virion. Each virion consists of a nucleic acid core, usually DNA or RNA, which contains the virus's genetic material. This core is encased in a protective protein coating called a capsid, and in some cases, the virion also has an outer envelope. These tiny infectious agents range in size from about 20–250 nanometers (nm) and are much smaller than prokaryotic cells.
Because they lack cellular structures like ribosomes and a metabolism, viruses are unable to reproduce or synthesize proteins independently and must rely on a host cell. Viruses have various shapes that are consistent and distinct for each viral family, and despite their simplicity, they can evolve and infect living cells.