vi´rus, n. [L., poison.] … 2. (a) any of a group of ultramicroscopic or submicroscopic infective agents that cause various diseases, as smallpox: viruses are capable of multiplying in connection with living cells…
The important parts of the definition are
“infective agents” and “capable of multiplying.” In terms of a computer, a virus is a program, or any executable code, which entered the computer unintentionally
(it infects), and spreads itself to other files and computers (it multiplies).
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In a broader sense but not as correctly, the term “virus” can mean any code which enters a computer without the operator’s knowledge and causes problems on that system. This definition also covers the so called Trojan Horses, Worms, and Time Bombs which enter a system and cause trouble, but may not replicate.
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New World Dictionaries, “Webster’s New Universal Unabridged Dictionary” (New York: Simon & Schuster, 1979), p. 2042