Security is protection from, or resilience against, potential harm (or other unwanted coercive ) caused by others, by restraining the freedom of others to act. Beneficiaries (technically referents) of security may be of persons and social groups, objects and institutions, ecosystems or any other entity or phenomenon vulnerable to unwanted change.

Security mostly refers to protection from hostile forces, but it has a wide range of other senses: for example, as the absence of harm (e.g. freedom from want); as the presence of an essential good (e.g. food security); as resilience against potential damage or harm (e.g. secure foundations); as secrecy (e.g. a secure telephone line); as containment (e.g. a secure room or cell); and as a state of mind (e.g. emotional security).

The term is also used to refer to acts and systems whose purpose may be to provide security (security company, security forces, security guard, cyber security systems, security cameras, remote guarding). Security can be physical and virtual.