What is Echelon or Project Echelon?

"Before PRISM there was ECHELON: the top secret surveillance program whose all-encompassing “dictionaries” (high-speed computers powered by complex algorithms) ingest and sort key words and text scooped-up by a global network of satellites, from undersea cables and land-based microwave towers."
https://www.globalresearch.ca/echelon-today-the-evolution-of-an-nsa-black-program/5342646

What is Echelon?
"A global network of electronic spy stations that can eavesdrop on telephones, faxes and computers. It can even track bank accounts. This information is stored in Echelon computers, which can keep millions of records on individuals.

Officially, however, Echelon doesn't exist."
https://www.theguardian.com/world/2001/may/29/qanda.janeperrone

"Project ECHELON has given the UK and United States (as well as other members of the Five Eyes) the capacity to track enemies and allies alike within and outside their states. The scope has evolved in that time period from keyword lifts in intercepted faxes to its current all-encompassing data harvesting."
https://techcrunch.com/2015/08/03/uncovering-echelon-the-top-secret-nsa-program-that-has-been-watching-you-your-entire-life/

"documents from the leaked Snowden files confirming the history of the fearsome ECHELON intercept project."
https://www.theregister.co.uk/2015/08/03/gchq_duncan_campbell/