What is Laser Threshold?

The question of "what is a laser" is closely related to that of what is laser threshold. We analyze possible definitions of laser threshold through the L-I curve and conclude that there is no consistent definition of a threshold when the spontaneous emission factor is large, as in the case of micro- and nano-lasers. All these definitions based on L-I curve are not consistent with the evolution of statistical properties of light output with pumping. Thus, we argue that the threshold definition based on L-I curve has to be abandoned for nanolasers, where the fraction of spontaneous emission into a lasing mode becomes quite large.