A virus signature is a self-replicating code that identifies a virus type. Each virus has its own signature. An antivirus program detects a virus type based on the virus signature.
A virus signature is a continuous sequence of bytes that is common for a certain malware sample. That means it's contained within the malware or the infected file and not in unaffected files. A characteristic sequence of bytes. Nowadays, signatures are far from sufficient to detect malicious files