Encumbrance is a three-dimensional assessment of mass, weight, and 
occupied space. Basically it's an assessment of how difficult something
is to carry around. i.e. 2-foot lead pole and a 10-foot wooden pole have
about the same encumbrance; the lead due mostly to weight; the wooden due
mostly to the awkward length.

Encumbrance affects spell casting, and is measured in "points" for lack
of a better unit.