I. pos ‧ te ‧ ri ‧ or 1 /pɒˈstɪəriə $ pɑːˈstɪriər/ BrE AmE adjective [only before noun]
[ Date: 1500-1600 ; Language: Latin ; Origin: 'further behind' , from posterus 'coming after' , from post ; ⇨ ↑ post- ]
technical near or at the back of something OPP anterior :
the posterior part of the brain
II. posterior 2 BrE AmE noun [countable]
the part of the body you sit on – used humorously SYN buttocks , bottom