亚 is a bound form (语素) that cannot stand alone as a word. It has two main uses: (1) as an abbreviation for Asia, as in 亚洲 (Yàzhōu 'Asia') and 东南亚 (Dōngnányà 'Southeast Asia'); (2) to mean 'second, inferior, or sub-', as in 亚军 (yàjūn 'runner-up, second place') and 亚热带 (yàrèdài 'subtropics'). You must attach it to another morpheme to form a word.