A telescope's pointing error (often abbreviated PE) is the angle between where in the celestial sphere the telescope is pointing, in terms of the image on the focal plane, and where it is intended to point. Pointing errors are measured as angles, e.g., in units of arcseconds.
A telescope always has a pointing error, the best that can be done is to keep it small enough not to undercut the validity of the observation. A modern research telescope is designed with a maximum-desired pointing error in mind, based upon the intended observations, and during design, the plan is analyzed to determine the sources that contribute, the size of the contribution and the resulting total pointing error. Sources include the same as for many forms of aberration, such as wavefront error, and include tolerances in the design of the parts, the effects of gravity and temperature on the equipment, atmospheric(seeing) effects.
The same phrase, pointing error, is used for non-astronomical optical and radio devices, including communications, e.g., to/from satellites, surveying equipment, and medical equipment.