Hello,
In the antivirus world, a signature is an algorithm or hash (a number derived from a string of text) that uniquely identifies a specific virus. Depending on the type of scanner being used, it may be a static hash which, in its simplest form, is a calculated numerical value of a snippet of code unique to the virus. Or, less commonly, the algorithm may be behavior-based, i.e. if this file tries to do X,Y,Z, flag it as suspicious and prompt the user for a decision. Depending on the antivirus vendor, a signature may be referred to as a signature, a definition file, or a DAT file.
A single signature may be consistent among a large number of viruses. This allows the scanner to detect a brand new virus it has never even seen before. This ability is commonly referred to as either heuristics or generic detection.
For Symantec AV Engine, read this Symantec Blog;