Refers to the part of a stem, tree trunk, pillar, or similar object where it meets the ground or surface.
A small mushroom was growing at the base of the tree.
The base of the pillar is slightly rotten.
Compound of 地 (ji, 'ground') and 際 (giwa, 'edge, verge'). The reading じぎわ follows rendaku voicing of きわ.