SageMath, the Sage Mathematics Software System, is a free software system for mathematics.
It was previously known as "Sage" and, even earlier, as "SAGE".
The shorter name, Sage, remains in use, when there is no ambiguity.
The longer name, SageMath, prevents confusion with other things called Sage (there are many).
Issue tracker: https://trac.sagemath.org/
See the wiki page From SAGE to Sage to SageMath for the history of the name.
Related projects include:
SageMath, Inc., a company aiming to raise money to fund SageMath development
For other things with similar names, but no relation, see the wiki page Other things called Sage.