Catalyst is an API specification developed for simulations (and other scientific data producers) to analyze and visualize data in situ.
Catalyst is committed to a stable ABI so that existing users can always use a newer version of the library. To this end, it is very strict about the API functions available through it. For example, Conduit is embedded, but only its C API is available (though it is rewrapped for easier use in C++).
Originally, Catalyst was part of ParaView. However, with ParaView version 5.9, the Catalyst API was revamped quite extensively and release as an independent project. This repository includes the definition together with a lightweight implementaton of this revamped Catalyst API.
The documentation describing the details, including build instructions and how to use Catalyst for in situ analysis and visualization in simulations is available on readthedocs.