It is either a 'solid-pane', or a 'panel' or a 'solid-panel' or an 'infill-panel' or a 'spandrel-panel' or just a 'spandrel'.
A pane is usually 'glass', but it may be obscure or a solid material too.
The latter 'spandrel' terms are the most 'architectural' terms - at least in the UK.
Its modern construction usage is not the same as its original meaning - referring to the triangular part of a wall above a stone arch - it was expanded last century to include any solid parts forming fenestration, and even the open parts in the sides of framed bridges...
Comme "panneau d'allège" ?
The prefix 'bottom' is only needed to clarify its location, or differentiate it from other panes/panels in the same frame..
A 'spandrel' like that can be located within multiple holed frame - at the bottom, at the top, or anywhere in between...
A 'spandrel' is more usually in a fixed frame.
If a similar infill is used in a hinged door or an opening window sash it is more likely to be referred to as a 'panel'.