Vortex-induced vibration is a nonlinear phenomenon that can damage buildings or produce energy. Here, the range of synchronization between fluid and structure is an important parameter... Using a coupled van der Pol and linear oscillator system, we derived an analytic, closed-form expression for the synchronization range as a function of fluid shedding frequency, or lock-in as it's also called. Our new definition is an improvement to an earlier synchronization definition as it includes the effect of dissipation; without it hysteresis is missing. The fluid speed range with hysteresis is found to be non-monotonic with forcing strength while the region of absolute synchronization increases monotonically. Our proposed synchronization definition can be a useful tool when designing structural dampers or energy harvesters but is a general solution for all systems that can be modeled with similarly coupled equations. (read more)