When someone says that a substance is soluble in another substance they mean that a certain substance is able to be dissolved in another substance. The units used for solubility is Molar solubility (moles/liter of solution before the solution is saturated).
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The definition of soluble is 'able to be dissolved'. If someone says that a substance is soluble in another substance, another way to phrase that is to say 'a substance is able to be dissolved in another substance'. The definition for solubility is 'the property of a solute being able to dissolve in a solvent'. Molar solubility measures how soluble (or how much of the solute can be dissolved in the solvent) the solute is in the solvent by measuring how many moles of the solute can dissolve in the solution before the solution is fully saturated.