Berkeley explains the distinction between immediate and mediate perception by using an example: "In reading a book, what I immediately perceive are the letters, but mediately, or by means of these, are suggested to my mind the notions of God, virtue, truth [or whatever the book is about]."

"mediate" perception	=	perception through (by means of) something else, not direct perception.
"immediate" perception	=	perception which is not mediate; direct perception.

Berkeley points out that we do not immediately perceive the causes of sounds, colors, etc. For example (not Berkeley's example), I do not immediately hear the truck coming down the street. I immediately hear the sound. My perception of the truck in this case is mediate.