IPA: /ˌʌndərˈsætʃəˌreɪtɪd/
KK: /ʌndərˈsætʃəˌreɪtɪd/
Not having enough of a substance mixed in, especially in a solution or mixture.
The solution was undersaturated, meaning it did not contain enough salt to dissolve completely.
Undersaturated is formed from "under-" (meaning below or insufficient) and "saturare" (from Latin, meaning to fill or soak). The word describes a state of being insufficiently filled or soaked, particularly in a chemical or physical context.
Think of being 'below' ('under-') the level of being 'filled' ('saturare') — that's why undersaturated means not fully filled.