Glossary entry

sensu stricto (s. str., or s.s.)
A Latin term meaning "in the strict sense". Often used in conjunction with a name when referring to the nominal taxon in the narrow sense of its subordinate nominotypical taxon (contrast with sensu lato (s. lat.)).

Source:
International Commission on Zoological Nomenclature (1999) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.