<p><strong>Noun</strong></p><ul><li>any of a class of compounds containing chemically combined water</li><li>in the case of some hydrates, as washing soda, Na₂CO₃⋅10H₂O, the water is loosely held and is easily lost on heating</li><li>in others, as sulfuric acid, SO₃⋅H₂O, or H₂SO₄, it is strongly held as water of constitution</li></ul><p><strong>Verb</strong></p><ul><li>to combine chemically with water</li></ul>