SageMath, the Sage Mathematics Software System, is a free and open-source software system for mathematics.
It was previously known as "Sage" and this shorter name remains in use when there is no ambiguity.
The longer name, SageMath, prevents confusion with other things called Sage (there are many).
Here are the main official Sage pages:
New Wiki: https://github.com/sagemath/sage/wiki
Old Wiki: https://wiki.sagemath.org/
The private company named "SageMath, Inc." is related but distinct from Sage. It provides the paid service CoCalc, an online environment for calculation and collaboration, with access to Sage and many other software.