Unsaturated Solution
Definition
A solution that could still dissolve more solute: below the saturation limit.
Technical definition
An unsaturated solution contains less solute than the maximum soluble at that temperature. Additional solute will dissolve, and no solid is in equilibrium with the solution.
Example
A teaspoon of sugar fully dissolved in a glass of water is unsaturated: you could stir in several more spoonfuls.
Common confusion
Unsaturated says nothing about concentration. A 5 M solution can be unsaturated if the solubility limit is 6 M: it only means below the maximum.