In our recent technical office hour a partner asked - What is a Catalyst?
Catalyst is a term we don’t actively use anymore, most likely you’ll find that internally we like to use
Catalyst to describe the entity data model.
Entities are a field in an item that include semi-structured metadata assigned to a body of text, however unlike facets the metadata are mapped to a specific span of text. This specific span of text might be a paragraph, a sentence, a word, or part of the word in the body which will be highlighted in the UI.
You can find more here: https://squirro.atlassian.net/wiki/spaces/DOC/pages/4161560/Item+Format.