The libpng license was derived in 1996 from the zlib license and has been referred to since by a variety of terms including at least the "zlib license" (even though it is different) and the "zlib/libpng license". Standards/free software bodies such as OSI currently refer to the license as the "zlib/libpng license".
The license differs most significantly from the zlib license in including a clause for compatibility with the UCITA. This was added around 2000 in response to the adoption of UCITA by the US state of Maryland.
Since this was after the adoption of the license by OSI questions have been raised about the actual OSI status