What is a dependent texture read?

Question

I've been reading papers on computer graphics, and every so often I come across the term "dependent texture read" or "dependent texture fetch" used in the context of querying textures in shader code. What is a dependent texture read, and what is the difference between this and a "normal" texture read?

Answer 1

The accepted answer (and the one below it) incorrectly restricts dependent texture reads to using the output of another texture read. That is incorrect.

A dependent texture read is any texture read with coordinates that are not the exact coordinates passed into the fragment shader of a program. Even a swizzle will cause a dependent texture read.

This is taken directly from Apple's OpenGL ES programming guide.