The Collaborative International Dictionary of English v.0.48:

Security \Se*cu"ri*ty\, n.; pl. Securities. [L. securitas: cf.
   F. s['e]curit['e]. See Secure, and cf. Surety.]
   1. The condition or quality of being secure; secureness.
      Specifically:
      (a) Freedom from apprehension, anxiety, or care;
          confidence of power or safety; hence, assurance;
          certainty.
          [1913 Webster]

                His trembling hand had lost the ease,
                Which marks security to please.   --Sir W.
                                                  Scott.
          [1913 Webster]
      (b) Hence, carelessness; negligence; heedlessness.
          [1913 Webster]

                He means, my lord, that we are too remiss,
                Whilst Bolingbroke, through our security,
                Grows strong and great in substance and in
                power.                            --Shak.
          [1913 Webster]
      (c) Freedom from risk; safety.
          [1913 Webster]

                Give up yourself merely to chance and hazard,
                From firm security.               --Shak.
          [1913 Webster]

                Some . . . alleged that we should have no
                security for our trade.           --Swift.
          [1913 Webster]

   2. That which secures or makes safe; protection; guard;
      defense. Specifically:
      (a) Something given, deposited, or pledged, to make
          certain the fulfillment of an obligation, the
          performance of a contract, the payment of a debt, or
          the like; surety; pledge.
          [1913 Webster]

                Those who lent him money lent it on no security
                but his bare word.                --Macaulay.
          [1913 Webster]
      (b) One who becomes surety for another, or engages himself
          for the performance of another's obligation.
          [1913 Webster]

   3. An evidence of debt or of property, as a bond, a
      certificate of stock, etc.; as, government securities.
      [1913 Webster]

   Syn: Protection; defense; guard; shelter; safety; certainty;
        ease; assurance; carelessness; confidence; surety;
        pledge; bail.
        [1913 Webster]

WordNet (r) 3.0 (2006):

security
    n 1: the state of being free from danger or injury; "we support
         the armed services in the name of national security" [ant:
         insecurity]
    2: defense against financial failure; financial independence;
       "his pension gave him security in his old age"; "insurance
       provided protection against loss of wages due to illness"
       [syn: security, protection]
    3: freedom from anxiety or fear; "the watch dog gave her a
       feeling of security"
    4: a formal declaration that documents a fact of relevance to
       finance and investment; the holder has a right to receive
       interest or dividends; "he held several valuable securities"
       [syn: security, certificate]
    5: property that your creditor can claim in case you default on
       your obligation; "bankers are reluctant to lend without good
       security" [syn: security, surety]
    6: a department responsible for the security of the
       institution's property and workers; "the head of security was
       a former policeman" [syn: security, security department]
    7: a guarantee that an obligation will be met [syn: security,
       surety]
    8: an electrical device that sets off an alarm when someone
       tries to break in [syn: security system, security
       measure, security]
    9: measures taken as a precaution against theft or espionage or
       sabotage etc.; "military security has been stepped up since
       the recent uprising" [syn: security, security measures]

The Free On-line Dictionary of Computing (19 January 2023):

security
computer security

    Protection against unauthorized access to, or
   alteration of, information and system resources including
   CPUs, storage devices and programs.

   Security includes:

   * confidentiality - preventing unauthorized access;
   integrity - preventing or detecting unauthorized
   modification of information.

   * authentication - determining whether a user is who they
   claim to be.

   * access control - ensuring that users can access the
   resources, and only the resources, that they are authorised
   to.

   * nonrepudiation - proof that a message came from a certain
   source.

   * availability - ensuring that a system is operational and
   accessible to authorised users despite hardware or software
   failures or attack.

   * privacy - allowing people to know and control how
   information is collected about them and how it is used.

   Security can also be considered in the following terms:

   * physical security - who can touch the system to operate or
   modify it, protection against the physical environment - heat,
   earthquake, etc.

   * operational/procedural security - who is authorised to do or
   responsible for doing what and when, who can authorise others
   to do what and who has to report what to who.

   * personnel security - hiring employees, background screening,
   training, security briefings, monitoring and handling
   departures.

   * System security - User access and authentication controls,
   assignment of privilege, maintaining file and filesystem
   integrity, backup, monitoring processes, log-keeping, and
   auditing.

   * network security - protecting network and
   telecommunications equipment, protecting network servers and
   transmissions, combatting eavesdropping, controlling access
   from untrusted networks, firewalls, and intrusion detection.

   Encryption is one important technique used to improve data
   security.

   OWASP is the free and open application security
   community.

   (2007-10-05)