A Nulled theme or plugin in WordPress is a pirated copy of a premium plugin or theme. This pirated copy is modified to remove all of the official brandings and distributed for free, or at a significantly reduced price. These are basically illegal.

Nulled WordPress plugins and themes are extremely dangerous for WordPress security. They may carry malware. The malicious code can spread across different files to disguise itself which makes it hard to detect and fix when our website is hacked. We may also lose our site data in the hack process.