Historically, TELEMAC and MASCARET have been developed for many years (decades) by different teams with different goals. As a result, people working mainly with TELEMAC carry on using the name TELEMAC, and the same for people mainly working with MASCARET.
Two or so years ago, the Consortium (mainly EDF-LNHE who own the two codes) decided to try to share expertise of both teams and work around common goals, in particular when it comes to the open source distribution and support of the codes. Hence the common code name open TELEMAC-MASCARET.
Note that you can access this site through
www.openetelemac.org or
www.openmascaret.org in exactly the same way.
Nowadays, the name TELEMAC does not represent one code but an ensemble of codes including SISYPHE, ARTEMIS, TOMAWAC, TELEMAC-2D, TELEMAC-3D, etc. The same is true of MASCARET that includes COURLIS etc. As it would be too long to name them all, we will probably stick with just open TELEMAC-MASCARET.
Hope this helps.
Sebastien.