Main Difference
The main difference between Century and Millennium is that the Century is a unit of time lasting 100 years and Millennium is a one thousand years.
Century
A century (from the Latin centum, meaning one hundred; abbreviated c.) is a period of 100 years. Centuries are numbered ordinally in English and many other languages.
A centenary is a hundredth anniversary, or a celebration of this, typically the remembrance of an event which took place a hundred years earlier.
Millennium
A millennium (plural millennia or millenniums) is a period equal to 1000 years, sometimes called a kiloyear. It derives from the Latin mille, thousand, and annus, year. It is often, but not always, related to a particular dating system.
Sometimes, it is used specifically for periods of a thousand years that begin at the starting point (initial reference point) of the calendar in consideration (typically the year “1”), or in later years that are whole number multiples of a thousand years after it. The term can also refer to an interval of time beginning on any date. Frequently in the latter case (and sometimes also in the former) it may have religious or theological implications (see millenarianism).
Century (noun)
A period of 100 consecutive years; often specifically a numbered period with conventional start and end dates, e.g., the twentieth century, which stretches from (strictly) 1901 through 2000, or (informally) 1900 through 1999. The first century AD was from 1 to 100.
Century (noun)
A unit in ancient Roman army, originally of 100 army soldiers as part of a cohort, later of more varied sizes (but typically containing 60 to 70 or 80) soldiers or other men (guards, police, firemen), commanded by a centurion.
Century (noun)
A Centuriate Assembly.
Century (noun)
A hundred things of the same kind; a hundred.
Century (noun)
A hundred runs scored either by a single player in one innings, or by two players in a partnership.
Century (noun)
A race a hundred units (as meters, kilometres, miles) in length.
Century (noun)
A banknote in the denomination of one hundred dollars.
Millennium (noun)
A period of time consisting of one thousand years.
Millennium (noun)
The period of one thousand years during which Christ will reign on earth (according to Millenarianist interpretations).
Millennium (noun)
A period of universal happiness, peace or prosperity; a utopia.
Millennium (noun)
The year in which one period of one thousand years ends and another begins, especially the year 2000.
Millennium (noun)
a period of a thousand years, especially when calculated from the traditional date of the birth of Christ
“silver first came into use on a substantial scale during the 3rd millennium BC”
Millennium (noun)
the prophesied thousand-year reign of Christ at the end of the age (Rev. 20:1–5)
“Archer’s treatise predicted that the millennium was close at hand”
Millennium (noun)
a utopian period of justice, peace, and prosperity
“the people must seize power—the millennium cannot be imposed on them from above”
Millennium (noun)
an anniversary of a thousand years
“the millennium of the Russian Orthodox Church”
Millennium (noun)
the point at which one period of a thousand years ends and another begins
“as we approach the millennium the only certainty is change”