Definition
Base of a shape
A base is a chosen side or flat region used as a reference for a shape.
What it means
The base is what the shape is measured from. It may look like the bottom, but “bottom” is not the rule. For a triangle or parallelogram, you can choose a different side as the base as long as the matching height is measured at a right angle to it.
Remember it
Choose the base first; then the height meets it squarely at a right angle.
Example
A triangle uses its 8 cm side as the base. Its perpendicular height to that side is 5 cm, so its area is ½ × 8 × 5 = 20 cm².
Watch for
The lowest-looking side is not automatically the base; the chosen reference and the matching perpendicular height must agree.
Common mix-up
For a prism or cylinder, both parallel matching ends are bases; for a pyramid or cone, there is one base opposite the apex.
Exact meaning
A base is a side or flat region used as a reference. Any side of a triangle or parallelogram may be chosen as a base; a prism or cylinder has two parallel matching bases; a pyramid or cone has one base opposite its apex.