A shape in 2-dimensional space - a flat shape.
There is no such concept in maths. A plane figure is a mathematical concept but that is not what the question is about.
A plain figure is a simple figure.
figure
in maths figure means "one of the elements that collectively form a system of numeration" for eg "0 & 1 are figures"
it is a flat side of a 3-d figure(solid figure)
a square is a plain figure which has all the sides equal but a rhombus is a plain figure which has the opposite sides equal.
Maths is everything. Maths can even be used by forensic scientists to create a figure of a whole body from a footprint. Anything that you can think of relates to maths in some form or another.
An octagon.
a triangle, a 3 sided figure, a figure with 3 points, etc.
a square that is a solid 3D figure
Octahedron
an octagon.