Q:

Why is the term "depressed" often used to describe a button which is pressed?

In several books that mention GUI, keyboard, or mouse buttons (e.g. the book Programming Windows by Charles Petzold), the authors refer to the state of a pressed button as depressed. Why is this term used instead of the word pressed, which has a simple and intuitive meaning? 
Some alternative terms that could be a better fit are pushed, clicked, or activated. As someone who is not a native-English speaker, the term depressed is unintuitive to me because it resembles the opposite meaning:  not pressed. 

A:

From TheFreeDictionary.com:

depression 1.
  a. The act of depressing.
  b. The condition of being depressed.

And the verb:

depress 4. to press or push down

So ... depressed works just fine for the state of a button being pushed in.

A:

"De" is also used as a prefix meaning "down to the bottom" or "away", which can also lead to "completely", as in the examples here: denude, denigrate.
It may count as an auto-antonym, also called contronym. But I can't think of an example of un-pressing something.

A:

Not sure, but my guess is that it's because technical writers are trying to make a distinction between the act of pressing something and the state of something being down/pressed/depressed.
depress and press are pretty much exact synonyms, but almost no one ever uses the term "depress" to describe the act of pressing something. It's correct, but the usage is rare.  It's usually used to refer to the state of something. The word pressed is used both ways.  But technical writers exploit the rarity of the usage of "depressed" to emphasize the distinction between between the state-word and the action-word.
So:

He pressed the button, and it was pressed.

Was the button actually in a down-state after it was pressed, or is the sentence just saying the same thing twice, that he attempted to press it?

He pressed the button, and it was depressed.

Okay, got it, his attempt to press the button was successful, and it left the button in a down-state.

