IPA: /mɪˈlɛniəm/
KK: /mɪˈlɛn.i.əm/
A period of one thousand years, often used to refer to a significant time in history or a future time of peace and happiness.
The year 2000 marked the beginning of a new millennium.
Millennium is formed from "mille" (meaning thousand) and "annus" (meaning year). The word refers to a period of a thousand years.
Think of 'mille' meaning thousand and 'annus' meaning year — that's why a millennium is a thousand-year period.