The number of bits used to define the shade or colour of each pixel in an image, a ‘bit’ being the smallest unit of memory or storage on a computer. (One ‘byte’ is eight ‘bits’.) A 1-bit image is black and white. An 8-bit image provides a 256-colour palette. A 24-bit image provides 16.7 million possible colours: a palette sometimes known as ‘True Colour’. A 32-bit image provides the same palette, plus an 8-bit greyscale alpha channel.