Zero-width Unicode refers to invisible characters such as zero-width space, zero-width joiner, and similar code points that are present in text but do not display in normal rendering. They can change how software parses a string while leaving the visible text unchanged, which creates a gap between what a person reads and what a program processes.
In cyber security, this matters because attackers can hide passwords, commands, file names, or other markers inside plain-looking text. That can help malware blend into configuration files, scripts, or messages reviewed by analysts. Defenders look for code-point-aware inspection, normalize suspicious input, and compare rendered text with raw bytes to spot hidden characters. In practice, zero-width Unicode is a concealment trick, not an exploit by itself, but it can make malicious content harder to notice and easier to automate.