The UKUSA treaty (pronounced You-koo-sa in intelligence circles) is the treaty set up after WWII to control the sharing of signals intelligence (SIGINT) between the USA and UK governments. Back then SIGINT was all about listening in on other governments' radio broadcasts, generally in the VHF (Very High Frequency) and UHF (Ultra High Frequency) wave bands, and using giant electromechanical "computers" to decrypt them.
The USA and the UK set up various listening stations around the world and put in place an infrastructure governed by a treaty, originally called BRUSA and then later UKUSA on how to share the effort of listening and decryption, and how to share the results.
There were in fact two versions of the treaty, a 1945 version and a major revision in 1948. Although they were very secret for many decades, the text of these agreements have now largely been made public and are available on both UK and US government websites, with a few bits still secret and blanked out.
The documents have a quaint old fashioned feel to them. In the 1945 version the UK is still referred to as the "British Empire". By 1948 this has been changed to a more politically correct "British Commonwealth". The minutes of meetings discuss such things as whether Washington is allowed to talk to the Canadians without checking with the British first. The position of the "London SIGINT board" is firm: all communication with the Canadians must come through London first - Canada belongs to us, don'tyaknow?
In fact Canada, Australia and New Zealand subsequently joined the treaty in their own right a few years later.
Although the text of these treaties are mainly public domain now, there are just a few bits which are blanked out. Some of these are obviously names, probably names of people who are still alive. Other parts are bit more mysterious. For example take Appendix K. It is completely new in the 1948 version and has no 1945 progenitor (unlike the other appendixes, and it must be said that most of the treaty is actually in its appendixes.) And every single word of the 30 pages of appendix K including its title is blanked out!
Which raises the question: what can have happened between 1945 and 1948 which necessitated a big new appendix, and is so sensitive that now 65 years later not a single word can be made public?