What does the term "riding" mean , as in West Riding of Yorkshire?

riding noun (often Riding when part of a name) 1 any of the three former administrative divisions of Yorkshire, East Riding, North Riding and West Riding.
ETYMOLOGY: Anglo-Saxon as thriding, from Norse thrithjungr: third part.
https://en.wikipedia.org/wiki/Riding_(country_subdivision)#Etymology

The "administrative division" sense of riding has been extended in Canada to describe political constituencies, as in the Member of the Legislative Assembly for the riding of Calgary-Foothills: "Giving the premier a wide lead over his challengers, Mr. Prentice's new riding of Calgary-Foothills has dutifully sent a Progressive-Conservative to the legislature in every election since the riding's creation in 1971." -from

theglobeandmail.com/news/national/alberta/jim-prentice-leads-pc-sweep-of-alberta-by-elections/article21347562 no longer exists

-see also http://www.tfd.com/riding (which indicates other places, like New Zealand, also use this sense)