[Haskell-cafe] meaning of "referential transparency"

[Henning Thielemann]

Can someone enlighten me about the origin of the term "referential 
transparency"? I can lookup the definition of "referential transparency" 
in the functional programming sense in the Haskell Wiki and I can lookup 
the meaning of "reference" and "transparency" in a dictionary, but I don't 
know why these words were chosen as name for this defined property.