millenniumphoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticPhoneticphonetic noun [C]
1. a period of 1000 years, or the time when a period of 1000 years ends
   The corpse had lain preserved in the soil for almost two millennia.
     How will you be celebrating the Millennium?
 

The Millennium Bug is a problem with older computers whose clocks are unable to recognise dates after 31 December 1999, with the result that programs might fail to work properly after this date.

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http://dictionary.cambridge.org/define.asp?key=millennium*1+0