How to correctly link a class with static member in c++

Question

Probably there is a really obvious answer to my question that I can't figure out.

I want to include to my main project a class containing a static member, but i get always a linker error. What I have done so far:

In the same folder I have main.cpp and element.hpp.
Code inside main.cpp:

#include <iostream>
#include "element.hpp"

int main(){
    element a(1);
    element b(2);
    std::cout << a(1) << std::endl;
    return 0;
}

and code inside element.hpp:

#ifndef _STATIC_HPP
#define _STATIC_HPP

class element{
private:
    static double lambda;
public:
    element(double const& l){ lambda = l; }
    double operator()(double const& x){ return lambda*x;}
};

#endif //_STATIC_HPP

To execute i use:

$ c++ -std=c++14 -c element.cpp;
$ c++ -std=c++14 main.cpp element.o -o exe
$ ./exe

What I expect to obtain is 2, but instead I get the following error message:

Undefined symbols for architecture x86_64:
  "element::lambda", referenced from:
      element::operator()(double const&) in main-f8deeb.o
      element::element(double const&) in main-f8deeb.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [exe] Error 1

I am becoming mad at this! Anyone any Ideas how to fix it?