It's short for Transatlantic. There's a Flyertalk Glossary page -- https://www.flyertalk.com/forum/glos...tterSelected=T
TPAC is short for Transpacific flights between US and Asia. Unlike US domestic fare filings, TATL and TPAC fare filings are asymmetric and depend on origin of trip. A one-way or roundtrip from the US to Europe will be priced using ex-US fare filings which are different than ex-EU fare filings which apply to one-ways or roundtrips which originate in the EU. So the cheaper US holiday season business class roundtrip fare filings only apply to roundtrips which begin in the US (which is not surprising since Europeans do not celebrate a number of these US holidays).