The high and growing penetration of inverter-based resources (IBR) in power systems challenges the way that system strength is assessed. It has been noticed that the standard indicator of system strength, short-circuit ratio (SCR), is not fully effective in anticipating the sub/super-synchronous oscillation phenomena that can arise from interactions of the control system of an IBR and the rest of the system. In this paper, system strength is first viewed from two perspectives, small-signal system strength and large-signal system strength, which is a natural separation considering the markedly different response of an IBR in normal operation and under faults. For assessing the onset of small-signal instability, a new metric for small-signal system strength is proposed and named Impedance Margin Ratio (IMR). IMR is the ratio between the allowed variation of the impedance of an IBR and the original value of the impedance at a particular point of interconnection. IMR is assessed at the oscillatory frequency of the dominant mode or taken as the minimum value across several modes. It is shown that as a localised frequency-domain indicator, IMR can successfully reveal weak points in the system towards various causes of oscillations.