The parser server created to convert between wikitext and VisualEditor-friendly formats is Parsoid.
Parsoid was developed as a key technical component of VisualEditor. Wikipedia’s traditional source language, wikitext, contains templates, references, tables, links, and other constructs whose meaning cannot be handled reliably by a simple visual editor. Parsoid bridges that gap by converting wikitext into an annotated document format that VisualEditor can manipulate, then converting edits back into wikitext when necessary.
This two-way conversion is more complicated than ordinary HTML editing. Wikipedia pages can contain nested templates, transclusions, unusual markup, and formatting conventions accumulated over many years. Parsoid was designed to preserve the relationship between the source and the rendered page so that visual edits would not unnecessarily destroy existing structure.
The name is associated with “parser” and “MediaWiki.” It is not a cache, search engine, or proxy: Varnish and Squid are caching or proxy technologies, while Lucene is an information-retrieval library. Parsoid’s work supported VisualEditor’s effort to make Wikipedia editing accessible without eliminating wikitext editing.