According to the
Merriam-Webster dictionary,
a hollow rectangular building block made of cement and coal cinders. Generally these are twice as long as they are wide, and equally wide as they are tall.
Cinder blocks typically have two vertical
holes in them, and
stack very easily. These blocks are used in
construction, featuring very
low-cost and some
insulating properties, but generally maligned because of their
ugliness.
Overhead view of a cinder block (the circles are the vertical holes):
_____________________
| ____ ____ |
| / \ / \ |
| | | | | |
| | | | | |
| | | | | |
| \____/ \____/ |
|_____________________|