Technological and scientific advances in neuroscience, information technology, and artificial intelligence have expanded how human and machine cognition can be targeted, manipulated, and weaponized. These developments have heightened the need for a coherent framework of Cognitive Security. Following review of current definitions, conceptualizations, and applications of Cognitive Security across military, academic, and organizational sectors, we propose a unifying definition of Cognitive Security as ‘the state of having trusted boundaries protecting cognitive assets against all forms of unauthorized influence or access’. Cognitive assets are defined broadly, encompassing individual cognitive processes, artificial intelligence systems, organizational decision-making structures, and neurobiological substrates. Drawing on this definition, we identify four interdependent cognitive resilience factors essential to maintaining Cognitive Security: (1) cognitive agility, the ability to adaptively manage biases, information processing, pursued goals, and decision processes; (2) machine psychology, safeguarding cognition-like functions in AI and ML systems; (3) neurosecurity, protecting human and non-human neural systems from biological or technological intrusion; and (4) systems engineering, the design of organizational and societal structures that foster resilience. We argue that effective Cognitive Security requires a holistic, multi-level approach that integrates these factors to defend against cognitive warfare, cyber-enabled influence operations, and emerging threats in human–machine ecosystems. By consolidating fragmented perspectives into a domain- and species-agnostic framework, this paper advances Cognitive Security as a distinct discipline and provides a foundation for research, policy, and practice aimed at strengthening resilience to ensure cognitive safety and superiority.