Definition of "botnet"

botnet

/ˈbɒtnɛt/

noun
plural botnets
1. (Internet, computer security) A collection of compromised computers that is gradually built up and then unleashed as a DDOS attack or used to send very large quantities of spam.
  Quotations
  Then they blacklisted addresses used by spammers. In response, senders started using botnets (networks of otherwise innocent computers).
  2010 November 20, “Long life spam”, in The Economist