T-tetromino

The  is a 4-polyomino.

It may be considered as an 8-sided figure with a single length 3 side, or as a 10-sided figure with colinear edges. The latter is most common when considering tessellations involving the as it makes it equilateral.

Related polytopes
The prism of the is the T-tetracube. It has two faces, and is the only tetracube with  faces.

The is the smallest of a general class of polyominos, the T-polyominos. The other members of this family can be formed by extending the central leg with additional squares while maintaining reflection symmetry.