noun
Refers to open space or the air above the ground, especially in set expressions involving something floating, hanging, or moving through the air. For outer space specifically, 宇宙 is more explicit.
風船が宙に浮いている。
The balloon is floating in the air.
強い風で帽子が宙を舞った。
The hat flew through the air in the strong wind.
noun
from memory; by heart
Used mainly in the fixed adverbial pattern 宙で, meaning to say or do something from memory without looking at notes. The synonymous そらで is also common.
See also: 空 (そら)
子どもたちは九九を宙で言えるようになった。
The children became able to recite the multiplication table from memory.
空 usually means the sky and also appears in そらで, 'from memory'; 宙 read ちゅう more often emphasizes midair or suspended space.
宇宙 means outer space or the universe; 宙 by itself often means the air or midair in everyday expressions.
暗記 is the act of memorizing something; 宙で describes producing it by heart after it has been memorized.
A Sino-Japanese noun using the kanji 宙, conventionally associated with sky, space, and midair. The development of the 'by heart' sense is not certain here, but it parallels the related expression そらで.