Which term describes a virus that changes itself every time it is executed?

A virus that changes itself every time it is executed is known as polymorphic. This term specifically describes a type of malicious software that employs various methods to mutate its code while keeping the original algorithm intact. As a result, each instance of the virus that infects a system may look different to antivirus programs and security defenses, which can make detection and prevention significantly more challenging.

Polymorphic viruses utilize techniques such as encryption and code obfuscation, enabling them to evade signature-based detection methods that many antivirus software rely on. Consequently, they can spread more effectively by avoiding detection during their execution.

In contrast, stealth viruses hide their presence by intercepting operations that antivirus programs use to detect them, while armored viruses utilize different techniques to make reverse engineering difficult. Multipartite viruses can infect multiple types of files and systems but do not inherently change their form upon execution. Thus, polymorphic accurately encapsulates the behavior of viruses designed to alter their code upon execution, making it the most fitting term.