TITLE: what is ≡ operator equal to in math?
QUESTION [7 upvotes]: Possible Duplicate:
When should I use $=$ and $\equiv$? 

I heard about this in our calculus class years ago. I was actually not in that class when the processor explained this. 95% of engineering student do not know about this operator. Trying to recall, what did it mean? And is it used standard in Math classes? I think it means approximately equal to. 
I am not 100% sure about the syntax.
Edit: Originally I asked for === operator

REPLY [2 votes]: It's used for various things in various contexts.  The one about "defined to be equal" is often rendered as ":=".  I haven't seen "$\equiv$" used for that.  It's certainly used for congruence with respect to a modulus; e.g. $44\equiv62 \pmod 6$, etc.  It's used for identities like $(x+1)^2 = x^2+2x+1$ when one wants to say that that is true for all values of $x$.
However, the variety of different uses that this symbol temporarily has in more advanced work has probably never been tabulated.