CoCalc (for "Collaborative Calculation in the Cloud") is an online platform which gives access via the web to virtual machines running a full Ubuntu GNU/Linux system, with lots of free software pre-installed, especially for mathematics and other sciences, including many programming languages (C, C++, Haskell, Java, OCaml, Python, Scala...), mathematics software systems (GAP, Macaulay2, Maxima, PARI/GP, R, SageMath, Singular), the Jupyter notebook with many kernels available), support for many markup languages (HTML, LaTeX, Markdown, TeX...), data analysis, machine learning and natural language processing software (NLTK, Pandas, R, Scikit-Learn, Statsmodels, TensorFlow), etc.
CoCalc also has collaboration features which lets users edit collaboratively any type of document (text, markdown, TeX/LaTeX, calculation worksheets...).
CoCalc also offers course management functionality, letting users create a class, send handouts, send and collect assignments, send out grades...
CoCalc is provided by the company "SageMath, Inc.", founded by William Stein to raise money in order to support the development of SageMath (the free software for mathematics that this wiki is about).
CoCalc was initially launched under the name SageMathCloud, and hosted at cloud.sagemath.com which now redirects to cocalc.com. In terms of domain names, sagemath.com is for the company "SageMath, Inc.", while sagemath.org is for the SageMath software.
To run an instance of CoCalc on one's own computer, the easiest is to use a Docker container.