"Shininess" word to indicate shine effect

In the shaders more than once met the word "shininess" to indicate shine of 3D-object, such as glass. Please tell me why this rare word is used? This word is not even in my dictionary. Thanks in advance!


I can't think of any good reason for the shift, but shininess has conclusively overtaken its synonymous rival glossiness in recent decades...

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To suggest that it's "not a word" just because it's not in some (or indeed, any) dictionaries is misguided, to say the least. A word is a word if people both use and understand it, dictionaries notwithstanding.