What is the philosophy behind Unity’s name
Something similar to this question. Why is Unity named Unity? What's the idea behind the name?
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
According to the Mark ShuttleWorth inaugural blog post about Unity, at the beginning Unity was designed to create an interface that would be usable on variety of machines (standard computers, netbooks, light versions)
A user could start his Ubuntu's experience on a tablet/netbook, then switch to his desktop and keep the same interface, Unity of the interfaces ;) .
Another reason may be the way that the applications are docked together. The runnable (launchers) and the running applications are merged in the same icons in the panel, that can be another way of Unity as well as the open Windows' toolbars being merged into the panel.
And last reason, as said on the unity website,
again, no matter the way you can control your machine, you'll find Unity in the way it is managed.
Unity refers to unified interface for various ubuntu devices, Eg. computers, tablets, mobile devices, tvs, etc.