bottomplate
noun
Plural: bottomplates
A layer, plate or board forming or reinforcing a bottom of a container, vessel, lake, etc·.
Examples:
Coating-based pans observably wear out with usage. Sooner or later, a lousy aluminium pan eventuates with a typical wavy bottomplate (there is rarely dependable stainless steel or cast iron underneath the overpriced lamination). (Source: J∙ Okram - Bartonian Potato Pancakes ● Cooking Recipe.)