After extended stirring of a spatula sized portion of a solid drug in 100 mL of water, the drug appeared to be insoluble. Yet when the liquid was separated from the solid and heated, solid residue was observed forming as the liquid evaporated. This evidence suggests that the liquid portion of the mixture formed after stirring the drug with water was

a saturated solution

Explanation:

A saturated solution is a solution that already contains just as much solute as it can normally hold at a given temperature.

When more solute is added to a saturated solution, the added solute does not dissolve.

The fact that when the liquid was separated and evaporated, some solid crystals were recovered means that the liquid has already dissolved the amount of solute that it can normally hold at that temperature. That is, the liquid is already a saturated solution hence more solute does not dissolve.