Orthographic projection or parallel projection is a form of projection that is sometimes used in 3D graphics to represent 3D objects on a 2D surface, such as a computer monitor. It works by scaling all the points on a 3D object down to zero in one direction (or axis, usually the z-axis). This has the effect of flattening the object onto the two axis which are perpendicular to the one which was scaled to zero (when scaling the z-axis, the x & y-axis would be perpendicular), thus producing a 2D representation of the 3D object.

It is often called a parallel projection...
"since the lines from the original points to their projected counterparts are parallel."
--- Dunn, Fletcher & Parberry, Ian (2003)

Orthographic project doesn't produce very realistic looking objects as it doesn't take into account the depth of an object, in real life objects appear to get smaller the further away from you they are (if you hold out a cube in front of you, the back of the cube will appear smaller than the front). With orthographic projection, a whole object will appear the same size regardless of its depth. To produce more realistic projections a perspective projection can be used instead, this type of projection takes depth into account.

References:
• Dunn, Fletcher & Parberry, Ian (2003) "3D Math Primer for Graphics and Game Development": Wordware Publishing Inc. ISBN: 1-55622-911-9