A polyomino can be defined as a plane geometric figure that is formed by one or more squad, edge to edge. The picture shows how it's done and you can use that to jog your memory. Still, if that definition doesn't remind you of anything, then explaining polyomino as a series of squares that you can arrange in the form of tiles (floor) should be better.
6 pairs
2 pairs
4 pairs
5 pairs
Triangles
Circles
Squares
Rectangles
9 cells
12 cells
10 cells
11 cells
1910
1907
1901
1900
Isaac Newton
Samuel Coulombs
Solomon Golomb
Andrew Pythagoras
Dimensions
Cubes
Equilateral triangles
Hexagons
Aeronautics
Astronomy
Statistical physics
Engineering
Equivalent polyominoes
Fixed polyominoes
One-sided polyominoes
Free polyominoes
Mirroring
Rotation
Glide reflection
Translation
Over watch
Minecraft
Fortnite
Sudoku