Entry 1 sense Webster, 1913 Hydromagnesite /(hī`drō̇*măg"nē̇*sīt)/ · Hy·dro·mag·ne·site · IPA 01 n. A hydrous carbonate of magnesia occurring in white, earthy, amorphous masses. 1. A hydrous carbonate of magnesia occurring in white, earthy, amorphous masses.(Min.)