Isometric Viewpoints refer to the camera angle used in many 2-Dimensional games to give the appearance that they are 3-Dimensional, without requiring 3D models.
Formal Definition:
True isometric projection uses 2-Dimensional art to create a 3-Dimensional appearance, with lines of perspective angled at precisely 60 or 120 degrees to each other. From the isometric viewpoint, a perfect cube takes the 2-dimensional shape of a perfect hexagon intersected at 120 degree angles.
Popular (yet incorrect) Use:
"Isometric" is commonly used to describe any elevated camera viewpoint that uniformly skews objects into a perceived sense of depth. These viewpoints can be along two (ex. seeing only the front and top sides) or three (ex. seeing the nearest three sides) dimensional axes.