Virus definitions are the computer coding that tells your anti-virus program what specific viruses (virii?) look like so that as the program scans your system it will recognize the virus when it sees it. Without current virus definitions, your anti-virus software will likely miss any newer viruses your computer may have contracted because it won't recognize them as viruses.
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Jim Bedsole, CRCM, CBA, CFSA, CAFP
My posts - my opinions