Computer security is the effort to create a secure computing platform, designed so that agents (users or programs) can only perform actions that have been allowed. This involves specifying and implementing a security policy. The actions in question can be reduced to operations of access, modification and deletion. Computer security can be seen as a subfield of security engineering, which looks at broader security issues in addition to computer security.
From : en.wikipedia.org/wiki/Computer_security

Security measures could include access control, virus protection, authentication tools such as digital signatures and encryption / decryption routines.