Security is the practice of protecting data, networks, programs and other information from unauthorized or unintended access, destruction or change. It involves a set of measures designed to ensure the confidentiality, integrity and availability of data and systems. In the context of JavaScript, security refers to techniques used to protect code, such as authentication, encryption, and access control. It also includes measures that prevent malicious code from executing, such as input validation, content security policies, and secure coding practices.