Question

what is mean by antimeridian

Answer 1

From 12'o' clock of midnight to 12'o' clock noon is denoted as a:m. After 12'o' clock of noon it will denote as post meridian.so abbreviation is a:m

Answer 2

Ante Meridian refers to the time before noon.

If you mean Anti Meridian , it is a longitude on Earth that is exactly opposite to the Prime Meridian .

In Short Your Answer is Anti Meridian - Any meridian (longitude) opposite to a particular meridian ( longitude).

Hope This Helps!