Confused about notation ":=" versus plain old "=" Relating to sets, I find the following in a text book: 
"...the set S := {1, 2, 3}".
The book has an extensive notation appendix, but the":=" notation is not included.
What exactly does ":=" mean, and how is it different from just "=", and how is it read?  Many thanks for any help.

The notation $A=B$ means $A$ is equal to $B$.  The notation $A:=B$ means "Let $A=B$."  It means you're saying what you will mean when you write $A$.
I suspect the $\text{“}{:=}\text{''}$ notation hasn't existed for more than about a half a century, so it's brand-new.
