What does cosh mean ? I mean , how is different from cos?
"cosh" is hyperbolic cosine.
The hyperbolic functions are based upon a hyperbola and not a circle.
A good explanation can be found in Wikipedia: https://en.wikipedia.org/wiki/Hyperbolic_function
Cosh is actually just the ordinary Cosine, though it's name is a little fancier: Hyperbolic Cosine. The same goes for the rest of the trigonometric notations.
cosh(x) is defined by
\(\displaystyle \cosh(x) = \frac{e^{x}+e^{-x}}{2}\).
It does have a relationship with the trig function cos,
\(\displaystyle \cosh(ix)= \cos(x)\).
+1491 
