A cold spot is any patch of internal surface that runs colder than the area around it. Because condensation forms wherever a surface falls below the room air's dew point, cold spots are where mould appears first and stays longest.
Common cold spots include: the top corners of external walls (where two cold surfaces meet), window reveals, the wall behind a wardrobe or sofa pushed against an outside wall (blocked from warmth and air movement), un-insulated concrete lintels and floor-to-wall junctions, and any thermal bridge through the structure.
Identifying cold spots is the first diagnostic step for a condensation problem. Feeling the wall temperature by hand, or using an infrared thermometer, quickly shows whether mould is following a cold pattern in the fabric — which points to insulation and heating, not occupant behaviour, as the cause.