Document

What is a .tei file?

A .tei file is a TEI P4 XML - Single Text File (version P4). This is a document format, so it holds text and layout meant to be read. Confusingly, this extension has been used by 4 different formats over the years — they're all listed below.

This extension has been used by more than one format

4 different formats have claimed .tei over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .tei file

Try LibreOffice (free, Windows/Mac/Linux) — it opens an enormous range of current and historic document formats.
Microsoft Word and Google Docs handle most mainstream document types.
If nothing opens it, open it in a text editor: many older document formats have readable text inside.

The registry records this extension against TEI P4 XML - Single Text File, TEI P4 XML - Corpus File, TEI P5 - Single Text File and others, so software from that family is the most likely match.

How to convert a .tei file

Once it opens in LibreOffice or Word, Save As .docx or .pdf to get it into a format you can use anywhere.

MIME type

When a .tei file travels over the web, servers label it with this MIME type:

application/tei+xml

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .tei.