This is the 5200th most frequent Russian word.
нижней
"Lower" or "bottom" (genitive case, feminine form in Russian).
Here, 'нижней' represents the lower level in a physical setting, indicating location and hierarchy.
Она живёт на нижней этажной площадке.
She lives on the lower floor landing.
In this sentence, 'нижней' refers to the relative position of a shelf, specifying its location as 'lower' among others.
На нижней полке я нашёл старую книгу.
On the lower shelf, I found an old book.
In this example, 'нижней' describes the boundary of an abstract concept—a diagram—implying its position in space.