In relief sculpture, the background plane or deepest field from which the subject emerges.
Fond is the rearward field or background of a relief. Its depth, flatness, curvature, texture, and relationship to projecting forms strongly affect legibility, shadow, and the apparent depth of the composition.
Etymology
French fond, 'bottom, ground, background.'
References
https://www.pierres-info.fr/glossaire_sculpture/index.html