What is ≡ operator used for in math? 
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 professor explained this. 95% of engineering student do not know about this operator. I'm trying to recall what it meant. 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.
 A: Since your professor was referring to engineering students, then it's likely they were referring to the identity symbol, which is used in an expression to mean the left and right hand sides are true for all values. So $\cos^2\theta +\sin^2\theta \equiv 1 $ since it's true for all $\theta$ whereas $\cos\theta = 1$ since it's true only for some.
A: There's the obvious meaning: Congruence modulo an integer, i.e. "$a \equiv b \pmod c$" (read: "$a$ is congruent to $b$ modulo $c$") which means $c \mid(b-a)$ ("$c$ divides $b-a$").
As someone else has mentioned it's also occasionally used to indicate equality of functions by writing $f(x) \equiv g(x)$, but why not just write $f=g$ then?!
A: 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.
A: The "≡" operator often used to mean "is defined to be equal."
