This is the 4213th most frequent Japanese word.
亜
"亜" means "sub-, secondary, or inferior," often used to indicate something that is next in rank or slightly off from the primary.
亜鉛 refers to zinc in chemical terminology, where '亜' modifies the term to indicate a specific substance.
私は亜鉛を含む食材を摂取するようにしています。
I try to consume foods containing zinc.
亜熱帯 signifies 'subtropical', where '亜' indicates 'sub-' or a lesser extent in geographical classification.
亜熱帯地域では様々な植物が見られます。
Various plants can be found in subtropical regions.
亜細亜 is a proper noun where '亜' is used to phonetically approximate the sound of 'A' in 'Asia'.