"PIN number" is a perfectly good name. Let's look at the official existing names: "personal identification number", which is a mouthful, and "PIN", which I think goes against two important psychological tendencies that people have. One is the desire to avoid confusion, which is in conflict with the fact that "pin" is an existing word with meanings unrelated to ATM transactions. Saying "PIN number" is clearer than just "PIN".
"I lost my PIN!"
"Did you say pen?"1
"Oh. Should I help you look for it? Is it sharp? Should I watch where I step?"
"No, you idiot..."
"I lost my PIN number!"
"Uh-oh. Were you able to get cash?"
"Yeah, but I had to go in and wait in line..."
The other tendency is to use full nouns to describe modern things, rather than just abbreviations.2 3 Nearly all important numbers that people have to deal with nowadays are "(something) numbers". There's your phone number, your account number, your flight number, your room number, etc. So when you go to the bank to get money, and you don't want to wait in line, what do you use? The machine. What machine? The ATM machine. And to use this machine, what do you punch into the keypad? Your number. What number? Why, your PIN number, of course!
Sure the N in PIN stands for "number". So what? It's just a bit of redundancy, which is not the evil force some people seem to think it is. In my study of foreign languages, I've found that many grammatical rules we think are set in stone are really just arbitrary. For example, double negatives. I'm sure you've heard, maybe even had, conversations like this:
"I don't know nothin'."
"So, you *do* know *something*?"
But in some languages, such as French, Spanish and Italian, double negatives are considered correct. Edith Piaf sang "Non, je ne regrette rien" (literally, "No, I don't regret nothing"), but nobody ever said to her, "Alors, vous regrettez quelque chose, oui?"
So, I don't think we should be too hard on people who say "PIN number" (or even "ATM machine", for that matter). "PIN number" is a colloquial term for the series of digits you enter when you need cash. The people who say it aren't stupid, they're just not all that technologically-minded, and they just want to call a number a "number", and a machine a "machine". And a spade a "spade".
And honestly, language is a communication tool, and I disapprove of using it as a game of "gotcha".
"I lost my PIN number!"
"Your personal identification number number? LOL!"
1. A perfect example of colloquial redundancy: in many parts of the South, people pronounce the words "pen" and "pin" the same, so they call a pen an "ink pen" to avoid confusion, even though pens use ink by definition.
2. Another good example is my answering machine. The manual calls it a "TAD" (telephone answering device). It only defines this acronym once, but then uses it obsessively throughout. Opening the manual to a random page, I can find stuff like:
- Once you turn on the TAD, it is set to answer calls (see "Setting the TAD to Answer Calls" on Page 16).
- You can also turn on the TAD remotely (see "Remote Commands" on Page 24).
3. There are abbreviations that people use by themselves without a full word involved, but these tend to be older and well-established, like "the USA" or "the USSR".