##X has never meant exit, but there's a reason for the confusion##

X has historically been overloaded to mean two different things:

1. Delete an item. For example:

   

2. Close or Dismiss a window. This is not the same as exiting an app but historically, hitting the X button almost always resulted in an application exiting, so that is why users sometime confuse the two:

• Historically, single-threaded operating systems and modally-oriented applications didn't have active background processes like Skype does, so when an application window was dismissed (not minimized with -), the logical thing to do was to exit the application.
• This is why the confusion has arisen over time (aka it's correlation not causation).
• Here are a few examples illustrating that even historically, X never meant Exit:
  • Historical versions of Microsoft Windows sometimes had X icons on dialog boxes to dismiss them.
  • Both historically and today, X is used for in-frame documents (e.g. in Microsoft Word) to close a document window but not to Exit an application.

##Today, X means the same thing##

When correctly used, the X meaning should still mean Close or Dismiss. For some applications, it makes sense for to exit the application when the window is closed/dismissed. For others (e.g. Skype, anti-virus firewall), it makes sense for the application to keep running in the background when the window is dismissed. So nowadays, Close does not always lead to an exit, but the X idiom is still the same.

##X has never meant exit, but there's a reason for the confusion##

X has historically been overloaded to mean two different things:

1. Delete an item. For example:

   

2. Close or Dismiss a window. This is not the same as exiting an app but historically, hitting the X button almost always resulted in an application exiting, so that is why users sometime confuse the two:

• Historically, single-threaded operating systems and modally-oriented applications didn't have active background processes like Skype does, so when an application window was dismissed (not minimized with -), the logical thing to do was to exit the application.
• This is why the confusion has arisen over time (aka it's correlation not causation).
• Here are a few examples illustrating that even historically, X never meant Exit:
  • Historical versions of Microsoft Windows sometimes had X icons on dialog boxes to dismiss them.
  • Both historically and today, X is used for in-frame documents (e.g. in Microsoft Word) to close a document window but not to Exit an application.
  • To address some unnaturally pedantic comments to this answer, here is a link to the Windows 95 interface guidelines which have many references to what the Windows Close Button is, how the term Close is defined in the Windows framework, and how the Close Button is used. In particular, the documentation refers specifically in many places to the close button being used to close windows (rather than exit the application). It also makes specific distinctions between Close and Exit behavior. For example, Page 108 references instances where the underlying application (the example they use is a printer) remains active after a window is closed...and includes specific guidelines to use the term Close instead of Exit.

##Today, X means the same thing##

When correctly used, the X meaning should still mean Close or Dismiss. For some applications, it makes sense for to exit the application when the window is closed/dismissed. For others (e.g. Skype, anti-virus firewall), it makes sense for the application to keep running in the background when the window is dismissed. So nowadays, Close does not always lead to an exit, but the X idiom is still the same.

##X has never meant exit, but there's a reason for the confusion##

X has historically been overloaded to mean two different things:

1. Delete an item. For example:

   

2. Close or Dismiss a window. This is not the same as exiting an app but historically, hitting the X button almost always resulted in an application exiting, so that is why users sometime confuse the two:

• Historically, single-threaded operating systems and modally-oriented applications didn't have active background processes like Skype does, so when an application window was dismissed (not minimized with -), the logical thing to do was to exit the application.
• This is why the confusion has arisen over time (aka it's correlation not causation).
• Here are a few examples of why even historically, X never meant Exit:
  • Historical versions of Microsoft Windows sometimes had X icons on dialog boxes to dismiss them.
  • Both historically and today, X is used for in-frame documents (e.g. in Microsoft Word) to close a document window but not to Exit an application.

##Today, X means the same thing##

When correctly used, the X meaning should still mean Close or Dismiss. For some applications, it makes sense for to exit the application when the window is closed/dismissed. For others (e.g. Skype, anti-virus firewall), it makes sense for the application to keep running in the background when the window is dismissed. So nowadays, Close does not always lead to an exit, but the X idiom is still the same.

##X has never meant exit, but there's a reason for the confusion##

X has historically been overloaded to mean two different things:

1. Delete an item. For example:

   

2. Close or Dismiss a window. This is not the same as exiting an app but historically, hitting the X button almost always resulted in an application exiting, so that is why users sometime confuse the two:

• Historically, single-threaded operating systems and modally-oriented applications didn't have active background processes like Skype does, so when an application window was dismissed (not minimized with -), the logical thing to do was to exit the application.
• This is why the confusion has arisen over time (aka it's correlation not causation).
• Here are a few examples illustrating that even historically, X never meant Exit:
  • Historical versions of Microsoft Windows sometimes had X icons on dialog boxes to dismiss them.
  • Both historically and today, X is used for in-frame documents (e.g. in Microsoft Word) to close a document window but not to Exit an application.

##Today, X means the same thing##

When correctly used, the X meaning should still mean Close or Dismiss. For some applications, it makes sense for to exit the application when the window is closed/dismissed. For others (e.g. Skype, anti-virus firewall), it makes sense for the application to keep running in the background when the window is dismissed. So nowadays, Close does not always lead to an exit, but the X idiom is still the same.