Polymorphism in cybersecurity refers to a technique where malicious software, such as viruses or worms, automatically changes its code or appearance each time it spreads or is executed. This constant alteration makes it difficult for traditional security tools, like antivirus programs, to recognize and block the malware based on known signatures. By continuously modifying itself, polymorphic malware can evade detection and persist longer within systems, posing a significant challenge for cybersecurity defenses.