What is Shared Socioeconomic Pathway?

A Shared Socioeconomic Pathway (SSP) is one of five narratives of how population, economies and technology might develop over the century. The emission scenarios of the assessment are labelled SSP$x$-$y$: $x$ names the pathway, and $y$ the approximate radiative forcing, in $\mathrm{W}\,\mathrm{m}^{-2}$, that the scenario reaches in 2100.