A slang term to describe Microsoft's true/false/error scheme used in many places in the Win32 API. It's used to describe when something has 3 states, as compared to Boolean, which has two states (usually true or false).
The GetMessage API is an interesting example of the bizarre Microsoft Troolean (as opposed to traditional, Boolean) logic. GetMessage is defined to return a BOOL, but the documentation specifies three types of returns, non-zero, zero and -1. I am not making it up! Here's an excerpt from the help file:

* If the function retrieves a message other than WM_QUIT, the return value is nonzero.

* If the function retrieves the WM_QUIT message, the return value is zero.

* If there is an error, the return value is -1.

Excerpt from Reliable Software, LLC
by ShinSan August 20, 2010

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