how to define "flat" or "walkable" terrain/objects?

Is there a property somewhere which defines the maximum angle which can be walked on? for example if 0 equals flat terrain and 90 equals exactly vertical (a wall perpendicular to the flat terrain) is it possible to define the angle above which an object cannot come to rest on a surface? e.g. setting this property to 45 would mean the player character could walk on any surface up to 45 degrees from horizontal but would slide off anything greater than 45?
I'm sure I've seen information on this somewhere before but can't find it now.