Vector Synthesis is a type of audio synthesis introduced by Sequential Circuits in the Prophet VS synthesizer during 1986. The concept was subsequently used by Yamaha in the SY22/TG33 and similar instruments and by Korg in the Wavestation.
Vector synthesis provides movement in a sound by providing dynamic cross-fading between (usually) four sound sources. The four sound sources are conceptually arranged as the extreme points of X and Y axes, and typically labelled A, B, C and D. A given mix of the four sound sources can be represented by a single point in this 'vector plane'. Movement of the point provides sonic interest and is the power of this technique. Mixing is frequently done using a joystick, although the point can be controlled using envelope generators or LFOs.