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A telescope's pointing error (often abbreviated PE) is the angle between where in the celestial sphere the telescope is pointing (in terms of what is actually imaged on the focal plane) and where the telescope 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. Research telescopes are now designed to keep pointing errors within some reasonable range, chosen as adequate and achievable. Such design includes modeling the pointing-error contributions of different portions of the telescope, of its mounting, etc., to assess in which parts, practical improvements can achieve the design-goal.
Contributors to pointing error include the same as cause many forms of aberration (such as wavefront error) and include tolerances in the design of the parts, and the effects of gravity and temperature on the equipment, and the atmosphere (refraction and seeing issues) adds to the effect.
The same phrase (pointing error) and concept are used for non-astronomical optical and radio devices, including communications, e.g., to/from satellites, surveying equipment, and medical equipment.