宙 (zhòu) is a bound form (b.f.) that appears almost exclusively in the compound 宇宙 (yǔzhòu, 'universe, cosmos'). In classical Chinese it referred to 'all time, past, present and future' — the temporal dimension of the cosmos, paired with 宇 (space). As a standalone character it is not used in modern Chinese; you will only encounter it as part of 宇宙 or related scientific/poetic terms like 宙斯 (Zeus, transliteration).