Discrete Cosine Transform
The Discrete Cosine Transform (DCT) is similar to the Discrete Fourier Transform (DFT): It transforms a signal from the time domain into the frequency domain. And just as the fourier transform uses sine and cosine waves to represent a signal, the DCT only uses cosine waves. The DCT I and II, is mostly used in image compression, while MDCT (DCT-IV) is used in audio encoding.
The DCT is an invertible, discrete orthogonal transformation. An orthogonal transformation consists of multiply the inner products of corresponding vectors or matrixes (including measures for angles and lengths).