"Sensu stricto" means "in the strict sense".  "Sensu lato" means "in the
broad sense".

For example,  Murex pecten is a Murex, sensu stricto.  It is in the actual
genus Murex.  Murex pomum is a murex, sensu lato.  It is actually a
Phyllonotus, mut it is a murex in the broad sense of ther word.