Plain English Meaning
An instrument approach into an airport that doesn't follow the usual design rules. Something about the airport -- mountains, obstacles, or limited airspace -- forced the procedure designers to build it differently than normal, and pilots need to know that before flying it.
Full Definition
An instrument approach procedure that contains one or more design elements outside the criteria of standard TERPS (Terminal Instrument Procedures) -- such as steeper-than-normal descent angles, higher-than-standard minimums, unusual missed approach paths, or course alignments that exceed normal tolerances -- typically because surrounding terrain, obstacles, or airspace prevent a conventional procedure from being built.