What is distance measured in degrees east or west of the prime meridian?

Answer 1

The Prime Meridian is defined as zero degrees of longitude.

The distance in miles between degrees of longitude depends on the latitude; one degree of longitude equals the cosine of the latitude * 60 nautical miles.

Note: If you're using Excel or some other spreadsheet program, be sure to convert degrees into radians before calculating the cosine of the latitude.

Answer 2

That angle is called 'longitude'.