Basic Definition

Rise is the distance from the crotch to the waistband.
edit

See Also

Super Low Rise Jeans
Low Rise Jeans
Mid Rise Jeans
High Rise Jeans
Gape
edit

References

Image courtesy of Atsui Tokyo, Measuring Guide - Denim and Pants http://shopatsui.blogspot.com/2008/09/measuring-guide-denim-and-pants.html
edit