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

botnet

    A large number of hijacked computers controlled by
   a botmaster via the Internet.  Some botnets have been
   estimated to include hundreds of thousands of computers.
   Botnets are sometimes rented out, sometimes for as little as 4
   cents per machine.

   The machines are recruited via a virus (e.g. sent by
   e-mail), a drive-by download or a worm.

   (2019-03-16)