A corner or nook — the small space at the edge or angle of a room or object.
猫が部屋の隅で丸くなって眠っていた。
A cat was curled up sleeping in the corner of the room.
箱の隅に古い手紙が見つかった。
An old letter was found in the corner of the box.
本の隅にメモを書いておいた。
I wrote a note in the margin (corner) of the book.
In Noh theater terminology, a stage position (the 'sumi' near the 'metsuke-bashira'); an old/specialized use.
能の舞台では、脇役が隅（すみ）に配置されることが多い。
On a Noh stage, supporting actors are often positioned at the 'sumi' (corner).