The term SPOOL was used simply because of it's function. Later on, the acronym was added to it. Thus, SPOOL is a backronym - as the acronym was added to merely fit the already-used term.

It is backronym.

The opposite of acronym....!!!!!

Or more appropriately use the terms expansion and long-form for opposites. =]

It was originally an acronym for Personal Home Page but is now a recursive "backronym" for PHP: Hypertext Preprocessor.

Which is a name written backward and used as pseudonym?

It's either an acronym or backronym depending on what you mean.

Acronym is when you get "FBI" from "Federal Bureau of Intelligence"

Backronym is when you get "Friendly Bee Incorporated" from "FBI"

If you mean what it's called when you take an existing word and give every letter a meaning, then I don't think there is a term for that.