A leaky acronym is an acronym used in partially expanded form, with the acronym itself left intact. The canonical example is “PIN number”: The “N” in “PIN” stands for “Number”, but the acronym has leaked and you’re really reading “Personal Identification Number number”. Some earth-dwellers call these “redundant acronyms” but Avaragado prefers to call them “leaky acronyms”, and so do you.
More specifically, in Avaragado’s superior terminology, “PIN number” is a Post-Leaky Acronym. It’s “Post” because the acronym leaks from the right-hand end. Another example is “WTS Server” (Windows Terminal Server Server).
You can also get Pre-Leaky Acronyms, such as “Acorn ARM” before the A was retconned into Advanced a hundred years ago. Avaragado is pretty sure there was a “Tarantella TSP” somewhere in the web site of a former employer.
Some Avaragado acolytes have spotted Mid-Leaky Acronyms: “Intel IA64 Architecture” leaks both the I (pre-leaky) and the A (mid-leaky).
The ultimate leaky acronym occurs when you have complete acronym breakdown, such as with the hugely leaky “Surface-to-Air SAM Missile”.
Other examples:
- Built on NT technology - NT stands for New Technology (allegedly). This particular example
- occurs in the Windows 2000 login screen.
- HIV virus - Human Immunodeficiency Virus virus. Probably as popular a leaky acronym as PIN number.
- ATM machine - Automated Teller Machine machine.
- LCD display - Liquid Crystal Display display
- TCP/IP protocol - Transmission Control Protocol/Internet Protocol protocol.
- HTTP protocol - Hypertext Transfer Protocol protocol.
- FAQ question - Frequently Asked Question question.
- RAID array - Redundant Array of Inexpensive Disks array. Ooh, mid-leaky!
- PERL language - Practical Extraction and Reporting Language language.
- PAC code - Porting Authorisation Code code.
Avaragado would be delighted to receive more examples to add to his collection.

My employers annoy me by referring to Lloyds TSB Bank – meaning that all of our official correspondence comes from Lloyds Trustee Saving Bank Bank.
Luckily, we appear to be collecting fragile financial institutions like a greedy rent boy collects STIs, so nowadays, the internal correspondence uses the overarching and more sensible moniker of LBG (Lloyds Banking Group).
There’s one in “LloydsTsb bastards”, the stupid bastards.