four-sided / quadrilateral / rectangular / square

square, rectangle

視角: visual angle / 資格: qualifications / 視覚: vision - sense of sight / 四角: square / 刺客: assassin / 死角: blind spot, dead angle

a box: a container with a lid / (archaic) a chamber pot for feces; (by extension) feces

qualifications

blind spot (area that cannot be seen) / blind spot (an inability to think clearly about a topic) / dead angle

Synonym of 暗殺者 (ansatsusha, “assassin”) / Synonym of 殺し屋 (koroshiya, “hitman”)

visual angle

box

原子核: atomic nucleus