(zoology, acarology) Any member of the subclass Acari (aka Acarina): thus, a mite or a tick; specifically, any mite of the genus Acarus.
2003, G. Leigheb, Mite bites, Andreas D. Katsambas, European Handbook of Dermatological Treatments, Springer, page 342,
Besides the acarus of human scabies (Sarcoptes scabiei hominis) (0.3-0.5 mm), many other acari which are parasitic on animals or which infest various plant species, foods, organic waste or soil may occasionally attack man (facultative parasitism vs. obligatory parasitism in the case of scabies).