When analogue equipment is pushed close to its limits it stops reproducing the signal perfectly. The result, called saturation, is a gentle form of harmonic and dynamic distortion: new overtones appear, peaks are rounded off and the sound thickens, qualities engineers describe as warmth or body. A transformer saturates when its magnetic core can no longer follow large signals, and magnetic tape behaves in a similar way.
Unlike the obvious grit of a guitar distortion pedal, saturation is usually applied in small amounts to drums, bass, keys or a whole mix, and plug-ins now model valves, tape machines, consoles and transformers. For jazz lo-fi producers it is a key ingredient for making clean digital recordings feel aged and cosy, often combined with filtering and with vinyl or tape noise.