the.com/solubility product

means It's the equilibrium constant for a sparingly soluble ionic compound dissolving in water — the product of its dissolved ion concentrations (each raised to the power of its coefficient) at saturation, a fixed value at a given temperature that tells you exactly how much solid will dissolve before the solution says enough.

from Not a word with a storied past but a term built from its job: 'solubility' comes via French from Latin solubilis, 'able to be dissolved,' itself from solvere, 'to loosen or untie' (the same root behind solve and dissolve). 'Product' is simply the arithmetic product — the multiplying together — of the ion concentrations. The concept itself grew out of 19th-century chemists applying the law of mass action (worked out by Guldberg and Waage) to sparingly soluble salts, giving dissolution its own precise equilibrium constant, usually written Ksp.