solubility
Solubility answers a simple question: how much will dissolve? If you keep adding sugar to a fixed glass of water, at some point no more will dissolve and the excess just piles up at the bottom. The amount that did dissolve, right at that limit, is the solubility.
Formally, solubility is the concentration of solute in a saturated solution at a stated temperature — the equilibrium point where the rate of drug dissolving exactly equals the rate of it crystallising back out. It is an equilibrium property and tells you the ceiling, not how fast that ceiling is reached. It is reported many ways: grams per litre, molarity, or pharmacopoeial descriptive terms ranging from “very soluble” to “practically insoluble”.
Solubility depends on temperature, the solvent, and for ionisable drugs strongly on pH, because the charged (ionised) form is usually far more water-soluble than the neutral form. Many modern drug candidates are poorly water-soluble, which limits how much can be absorbed; formulators counter this with salt formation, cosolvents, surfactants, complexation, or particle-size reduction. Intrinsic solubility specifically refers to the solubility of the un-ionised form.
Ibuprofen has an aqueous solubility of only about 0.02 mg/mL at neutral pH but dissolves far more readily as its sodium or lysine salt, which is why some fast-acting products use a salt form.
Salt forms can raise a poorly soluble drug's effective solubility.
Solubility (how much) and dissolution rate (how fast) are different properties. A drug can have low solubility yet dissolve quickly if milled to fine particles, or high solubility yet dissolve slowly if formulated as a large dense tablet.