[This is a fast-access FAQ excerpt.]
Disputes occur about the legitimacy of placing after an acronym/
initialism the last word that is abbreviated in it, e.g., "AC
current", "the HIV virus". "AC" and "HIV" by themselves will
certainly suffice in most contexts. But such collocations tend to
become regarded as irreducible and uninterpretable words. "The
SNOBOL language" and "BASIC code" are as good as "the BASIC
language" and "SNOBOL code"; and why should "an LED display" (Light
Emitting Diode display) be reasonable, but not "an LCD display"
(Liquid Crystal Display display)? The extra word may guard against
ambiguity; e.g., "I've forgotten my PIN" might be mistaken in
speech as being about sewing, whereas "I've forgotten my PIN
number" identifies the context as ATMs.
It cannot be denied, though, that many such repetitions stem
from ignorance. The more familiar someone is with computer memory,
the less likely he is to say "ROM memory" or "RAM memory".