# Discrete Fourier Transform: What is the DC Term really?

I am currently toying around with the Discrete Fourier Transform (DFT) in Matlab to extract features from images. I like to fully understand the concepts that I use. I have read several explanations, such as this, but so far, none really explained the meaning of the "DC term". All I know is that the k'the term of the DFT can be written as:

$F_k=\sum_{j=0}^{N-1}{x_j\omega^{k\,j}}$ where $\omega$ is the twiddle factor.

That means that the first term (the DC term), $F_0=\sum_{j=0}^{N-1}{x_j}$, is an amplitude without frequency.

Can someone explain why is it called the DC term? What is it's relation to "Direct Current"? And what is the relevance of the DC term? When is it useful, and for what?