楔形的英文叫wedge，这是一个什么样的几何体呢？

且看维基百科的解释：

In solid geometry, a wedge is a polyhedron defined by two triangles and three trapezoid faces. A wedge has five faces, nine edges, and six vertices.

A wedge is a subclass of the prismatoids with the base and opposite ridge in two parallel planes.

A wedge can also be classified as a digonal cupola.

Comparisons:

A wedge is a parallelepiped where a face has collapsed into a line. A quadrilaterally-based pyramid is a wedge in which one of the edges between two trapezoid faces has collapsed into a point.

楔形大致就上面图中的样子，两个三角形和三个四边形，不过要求四边形为梯形，实际上要求满足其中三条边是平行的，哪三条边呢？就是图上看到的是水平的那三条边。这是硬性条件，否则不能称之为楔形。

三棱柱是特殊的楔形。

图中的底边是一个矩形，这只是一个特例，并不是所有的楔形都要求有一个四边形为矩形。