The rendered fat of (usually) a pig. It is white and solid, or semi-solid. It has many uses, mostly in cooking (can be used as shortening, to fry food, in gravy etc). In addition, it is one of the main ingredients for soap (along with ashes for the lye content).
One documentary on the creation of Stone Henge suggests that lard may have been used to lubricate tracks upon which the large boulders could have been pulled up the hill upon which they stand.
See also: Ghee