Transformations are mathematical operations that can be applied to an object to change its scale, position and orientation.
Transformations are the mathematical operations that can be applied to an object (often a set of vertices that belong together) to move it, rotate it, or otherwise transform it.
Transformations are usually applied using a matrix.
Transformations are not necessarily bijective; data may be lost, for example, in a projection from a 3D object to it's 2D representation on a screen.
