There is no taxonomic rank corresponding to sardine. The name is applied across the family Clupeidae and its relatives to small, oily, schooling fishes of a suitable size for canning. In the northeast Atlantic and Mediterranean it points to Sardina pilchardus; in the Pacific and southern hemisphere it commonly means one of the Sardinops populations; elsewhere sprats and round herrings appear under it.
Size does much of the work in the naming. In British usage the same species is a sardine when small and a pilchard when large, which is a trade convention rather than a biological distinction. Regulatory codes in some jurisdictions specify which species may be labelled sardine and require a qualifying country or species name for others.
The consequence is that catch and consumption statistics collected under the common name can conflate distinct stocks with different dynamics. Sardine-type populations are famous for large multi-decadal swings, and whether those regime shifts are driven mainly by ocean conditions or amplified by fishing is one of the long-running debates in fisheries science.