How to fix "Undefined Reference to 'Timer::Infinite'"

Question

I am having a problem with referencing a variable from another header file.

public:
    static const long Infinite = -1L;
    Timer(){}
    Timer(const std::function<void(void)> &f) : funct(f) {}
    Timer(const std::function<void(void)> &f, const unsigned long &i, const long &repeat = Timer::Infinite) : funct(f), interval(std::chrono::milliseconds(i)), CallNumber(repeat) {}

when I try to use 'Infinite' in the other header file as follows

Timer MailTimer(TimerSendMail, 2000*60, Timer::Infinite);

I found error saying "undefined reference to `Timer::Infinite'|"