aqueous solubility

Aqueous solubility is how much of a compound will dissolve in water before the rest stays as undissolved solid. Like adding spoon after spoon of sugar to a cup of tea, at some point no more goes in and a sludge collects at the bottom; that saturation point is the solubility.

For a drug this is foundational: a molecule must dissolve before it can be absorbed, reach the bloodstream, and act. Poorly soluble compounds may pass through the gut largely undissolved and unabsorbed, or fail in assays simply because they precipitate out of the test solution and give misleading results.

Solubility depends on the compound's lipophilicity, its ionization at the relevant pH, and how tightly its crystal lattice holds the molecules together. Greasy, flat, high-melting molecules tend to be poorly soluble. Chemists improve solubility by adding polar or ionizable groups, breaking up flat rigid cores, or selecting a more soluble salt or solid form.