This is a general term. This upper definition is as follows.
A method of downloading a program to a computer using a few initial instructions that allow you to enter the rest of the program from an input device.
In short, this is a way to start by loading the outline of a set of programs, and then pull out other dependencies to get started with this program / framework (in your case, the angularjs project).
And I didn't look at the Google documentation (unless you look at the code school, and they don't seem to be related to twitters bootstrap at all).
This question seems to be discussing it in detail , the definition is related to "pulling out one of their boot files"
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