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A polymorphic virus is defined by its ability to change its signature or code each time it infects a new system. This characteristic is significant because it enables the virus to evade detection by antivirus software, which relies on recognizing known signatures or patterns of malicious code. By altering its appearance while maintaining the same core functionality, a polymorphic virus can effectively bypass security measures that are designed to identify and quarantine specific threats.

The ability to change makes polymorphic viruses particularly dangerous, as they can replicate and spread without being easily identifiable. This adaptability contrasts sharply with other types of malware, such as those that remain static upon infection, which can often be detected more straightforwardly using signature-based detection mechanisms.

Other options provided don't accurately describe a polymorphic virus; for instance, viruses that only target email systems or utilize social engineering tactics are describing different behaviors and methods of propagation that are not relevant to the polymorphic nature of the virus.