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.