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It is borrowed from computer programming: it means that the item on the left hand side is being defined to be what is on the right hand side. For example, $$y := 7x+2$$ means that $y$ is defined to be $7x+2$.
This is different from, say, writing $$1 = \sin^2(\theta) + \cos^2(\theta)$$ where we are saying that the two sides are equal, but we are not defining "1" to be the expression "$\sin^2(\theta) + \cos^2(\theta)$".
Basically, some people think that there should be notational difference between saying "I define
blah to be equal to
blankety" and saying "
blah is equal to
blankety". So they use
:= for the first and
= for the latter. Usually, it is clear from context which of the two uses of the equal sign is intended (often because of signal words like "Let", "We define", etc.)
I think the Bourbaki used it first.. not sure.. I know physicists use $\equiv$