Security refers to protecting aircraft, passengers and crew from deliberate attacks or sabotage
Security is a very broad topic and covers all measures taken on the ground and in the air to protect aircraft, passengers and crew from sabotage, hijacking or other deliberate, malicious attacks.
Note that security should not be confused with safety, which is concerned with protection against accidents and other unintended dangers. Some questions may concern both safety and security.
Use this tag for all questions about how aviation security is measured, achieved and improved, but note that some information - such as details of cockpit and crew protocols - may be not be public and it may even be illegal to make it public.
Questions that are purely about ground security measures (e.g. passenger documents, identification and background checks, luggage screening) are often off-topic unless there is some very direct relevance to actual flight operations; in those cases you may get a better answer on travel.SE.