Draw a straight line and in the middle mark the zero position. Now make even marks leading away from that point. Call one direction positive numbers and the other negative numbers. Usually horizontal lines have positive numbers going to the right and vertical lines have them going up. Conversely down or to the left are negative numbers.
You would call it million.
Most mathematicians would call that a 1000-gon. If you want to show off, call it a "chiliagon."
you call it a prime number
you would call this # to the fourth power
A three-hundred-year celebration is called a tricentennial.
Celebrating 25 years of marriage is called the Silver Anniversary.
there celebration call the corn dance
There is no difference, but birthday would definitely be the more widely accepted label. The word "birthday" itself is elegant - it accurately gets the point across with four less syllables than birth anniversary.
A 100 year celebration is called a centennial celebration.
the 100th metre is the hectometre
Octocentennial or octocentenary.