I'm asking about when the term is used in context of software UI.
I see Wikipedia articles related to hardware when Googling the term, but not sure what people mean when they refer to it in context of software/mobile UI/web UI.
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Using the term 'form factor' is not usual for software UI, as you ask. But why not? As for me, 'form factor' stands for 'size', 'dimension', 'footprint', etc. So if you tell me about your application form factor I think about screen resolution and screen dimensions - i.e. screen form factor. But as we can see this is again about hardware. The problem with software UI that form factor is static from its nature and software UI is flexible in general. So only some characteristics of software UI which depends on corresponding hardware characteristics may have form factor, but not software or application in common.
Its refers to the physical dimensions of the hardware in the context you have mentioned
This simple link can solve the mystery http://en.wikipedia.org/wiki/Form_factor