Polyominoes is the general name given to plane shapes made by joining squares together. Note that the squares must be 'properly' joined edge to edge so that they meet at the corners.|
Each type of polyomino is named according to how many squares are used to make it. So there are monominoes (1 square only), dominoes (2 squares), triominoes (3 squares), tetrominoes (4 squares), pentominoes (5 squares), hexominoes (6 squares) and so on.
Though the idea of such shapes has been around in Recreational Mathematics since the beginning of the 1900's, it was not until the latter half of the century that they became known to a wider audience.
In 1953 Solomon W Golomb (an American professor) first introduced their names and outlined their possibilities to mathematicians, who seized on them with considerable interest.
They were not brought to the notice of the world in general until 1957 when Martin Gardner (in his famous column in the Scientific American) wrote about them, and they have remained a rich source of spatial recreation ever since.
Pentominoes (made from 5 squares) are the type of polyomino most worked with. There are only 12 in the set (because shapes which are identical by roatation or reflection are not counted). This means that they are few enough to be handleable, yet quite enough to provide diversity.