Technically it's what is called an "Geoid"[0] but for most purposes the description of an "oblate spheroid" [1] will do. Those "shortest straight distances" are called "great circles" or geodesic; an arc between two points on a spheroid also called a great distance. Technically, because the Earth isn't spheroidal but oblate, the shortest distance isn't quite a great circle. [2] The maps used for aeronautical navigation are "gnomonic" in projection to show great circles as straight lines. [3]
Technically it's what is called an "Geoid"[0] but for most purposes the description of an "oblate spheroid" [1] will do. Those "shortest straight distances" are called "great circles" or geodesic; an arc between two points on a spheroid also called a great distance. Technically, because the Earth isn't spheroidal but oblate, the shortest distance isn't quite a great circle. [2] The maps used for aeronautical navigation are "gnomonic" in projection to show great circles as straight lines. [3]
[0] http://www.kartografie.nl/geometrics/reference%20surfaces/re...
[1] http://en.wikipedia.org/wiki/Oblate_spheroid
[2] http://en.wikipedia.org/wiki/Great_circle
[3] http://en.wikipedia.org/wiki/Gnomonic_projection