A citation is a reference to a source, especially in a scholarly work.
Wikipedia style:
Russell, A.P.; Baskerville, J.; Gamble, T.; Higham, T. (November 2015). "The evolution of digit form in Gonatodes (Gekkota: Sphaerodactylidae) and its bearing on the transition from frictional to adhesive contact in gekkotans". Journal of Morphology. 276 (11): 1311โ1332. doi:10.1002/jmor.20420. PMID 26248497.