Referential transparency is an oft-touted property of (pure) functional languages, which makes it easier to reason about the behavior of programs. As there is no single formal definition, the one Christopher Strachey used to introduce the term into the study of programming languages will have to suffice:
Side effects like (uncontrolled) imperative update break this desirable property. C and ML are languages with constructs that are not referentially transparent.
As an example, consider the following program in Standard ML:
puts "h"; puts "a"; puts "h"; puts "a"
"haha". In an attempt to factor out the repetition, we write
let val x = (puts "h"; puts "a") in x; x end
but now the laugh is on us, because
"ha" is only printed once. The reason is that
puts's side effect is only realized when
x gets bound, so we should have written
let fun x () = (puts "h"; puts "a") in x (); x () end
Haskell's monadic I/O system distinguishes between values and
actions like the
puts procedure above. So we do indeed have that
putStr "h" >> putStr "a" >> putStr "h" >> putStr "a"
is equivalent to
let x = putStr "h" >> putStr "a" in x >> x
 Wolfram Kahl provides a USENET post by Tom DeBoni containing a summary of various definitions for referential transparency.
 From Fundamental Concepts in Programming Languages, 1967 (page 9 of 39).
puts can be defined as
fun puts s = TextIO.output(TextIO.stdOut, s);
 There is some debate about whether the imprecisely-defined semantics of
Int breaks referential transparency. For instance,
even (maxBound :: Int) may be
True in some contexts and
False in others. Another example is
System.Info.os :: String.
 One perspective is that Haskell is not just one language (plus
Prelude), but a family of languages, parameterized by a collection of implementation-dependent parameters.
Each such language is referentially transparent, even if the collection as a whole might not be.
Some people are satisfied with this situation and others are not.