The speed of change in the world of cybersecurity is amazing. If you attend a meeting in which cybersecurity is discussed, it can be like listening to a foreign language, leading you to ask what on earth people who use terms like blue team, black hat, fileless malware, and quantum cryptography are talking about. This dictionary began life as the reference section at the back of my first book, Cybersecurity for Beginners, and has taken on a life of its own as a highly-regarded source that defines hundreds of the primary cybersecurity terms. It is designed to be a useful companion for anyone who wants to keep up with cybersecurity terms or confound others with their understanding. This fourth edition includes over 100 additional terms to reflect the evolving trends in cybersecurity, cybercrime and social engineering. Finally, cybersecurity does not need to sound like a different language. This clear and concise dictionary of cybersecurity terminology is suitable for both experts and laypersons.