WordNet (r) 3.0 (2006):

spyware
    n 1: computer software that obtains information from a user's
         computer without the user's knowledge or consent

The Jargon File (version 4.4.7, 29 Dec 2003):

spyware
 n.

    1. Software which, when installed by a user insufficiently enlightened to
    avoid it, enables third parties to snoop the user's hard drive or monitor
    their network transactions. Though the term seems to have entered use in
    the late 1990s, it achieved real popularity as applied to Microsoft Windows
    XP. Some back door features in XP permit Microsoft to (for example)
    covertly scan your disk directories for the names of files it might deem to
    be warez.

    2. Systems for spying on email and web traffic, such as the FBI's
    Carnivore.


The Free On-line Dictionary of Computing (30 December 2018):

spyware
sypware

    (Or "adware") Any type of software that transmits
   information without the user's knowledge.

   Information is sent via the Internet to a server somewhere,
   normally as a hidden side effect of using a program.
   Gathering this information may benefit the user indirectly,
   e.g. by helping to improve the software he is using.  It may
   be collected for advertising purposes or, worst of all, to
   steal security information such as passwords to online
   accounts or credit card details.

   Spyware may be installed along with other software or as the
   result of a virus infection.  There are many tools available
   to locate and remove various forms of spyware from a computer.

   Some HTTP cookies could be considered as spyware as their
   use is generally not made explicit to users.  It is however
   possible to disallow them, either totally or individually, and
   some are actually useful, e.g. recording the fact that a user
   has logged in.

   (http://spychecker.com/spyware.html).

   (2004-05-23)