The company I work for (based in the UK) is building a piece of software for iOS, and they use "Dismiss" instead of "Close", "Exit", "Cancel", etc. for the button to close the page at hand.
I find the word "Dismiss" a bit too formal, and rather unusual. Is it UK specific? Is it iOS specific? Am I the only one who finds it a bit weird?
Depending on the context in which the button appears, I would use the word Done or Close or Cancel or Exit but never Dismiss.
Close would be used if the user first Opened something which does not involve change. Eg A message, a photo, T&C, Product Information etc. Close could also be used as a form of Accept to mean close and accept any changes, although the term does not really clarify that acceptance strongly enough in my opinion. Either way, Close should pair with the matching Open in the user's mind.
Cancel would be used if the user initiated a process which involves change. The concept of canceling applies whether an actual change has taken place already or not. As opposed to simply reading a message for example.
Exit would be used if closing down or backing out of scope of the current application.
The only one in which either Dismiss or Done could apply to here as alternative micro-copy here, is the Close option.
Close or Done, you could argue either way, and context and surrounding copy plays a part in deciding this, but Dismiss I would leave well alone as it's almost certainly a system model term accidentally used by a programmer who failed to use appropriate micro-copy for the target audience's user model.
Dismiss is certainly unusual and therefor to be avoided.
The only thing that comes close is a "cancel" button I sometimes see. E.g. when you make a message in the facebook app, you can choose "cancel" or "post".
dismiss is actually pretty common. It 'send away' and often used for informational popups where close or ok could be misconstrued. It is somewhere between close and finish, the difference is subtle but there.
"Dismiss" seems perfectly natural to me (a Brit), as long as it's referring to a single dialogue box, like a notification of some kind. It'd be odd to use it for a whole app, or a complex screen within an app. I can certainly imagine casually saying "I dismissed the dialogue box", meaning similar to "I closed the dialogue box".
"Exit" would sound odd on a single notification. It sounds more like you're leaving the whole application, or some major part of it.
"Close" can work in either case. Which means that you're always going to be OK using "Close", but you might lose any cues as to whether this button will close a small part of the screen versus the whole screen.
Text editors and web browsers usually use "Close" to mean "I'm done working with this one document/tab/page", versus "Exit" to mean "I'm done working with all these documents - close the whole app".
I've just discovered also that ux.stackexchange also uses the word "Dismiss": 
It does depend what the target market is for your app.
If it's say a 'World of Jane Austen' App then 'dismiss' might fit the overall look and feel.
Ooooh no...Dismiss sounds far too stern. Close is perfectly fine. One wonders what someone is trying to convey with the word 'dismiss'....unless of course it is an Army application
I like 'OK' or 'Close'.
Had a discussion with a colleague about the correct copy for a button to close a system alert. His thoughts were that 'OK' was wrong because it implied that the user should consent to something she really had no control over (e.g: "The system was unable to perform the desired action. OK?". No, that's really not OK). He felt that 'Close' was better because that was all the user could do with the alert.
I for my part like 'OK' because it can also be interpreted to mean 'I understand'.
This can be tough. No matter how simple you try to keep things, it's not always so easy...
If I'm using an app and a new window opens for secondary process, id expect to "close" to return to the primary app.
If I'm using an app and now I'm done, I'd expect to click "x" or "exit".
With regards to navigation... "back" to go back to the page previously navigated from... "Cancel" when in the middle of some workflow... and "ok" when at the end of a workflow.
There are a million other scenarios that don't quite fit with the options above but you have to use the 80/20 rule and use the closes match. You may have the best of intentions, however one exception will lead to another and soon you'll be providing your users with an inconsistent and uncertain experience.
--Josh
