Solvents are dissolving agents. Diluents are diluting substances.
When a synthetic aroma chemical is in powder form, a solvent is required to convert it to a liquid for use in fragrance oil. Solvents help the components of a fragrance burn more easily, blend better, improve fragrance throw, and increase fragrance longevity.
If an essential oil is in a highly concentrated form, a diluent is required to thin it so that it will remain suspended in the liquid fragrance oil. This substance adjusts the density of fragrance oils so that it's easier for the wax/fragrance mixture to be pulled up through the wick to be burned at the flame.
Some diluents are used to "cut" fragrance oils after they are produced so that an expensive fragrance oil can be sold at a lower price with a weaker strength. Once CandleScience fragrances are developed, they are NEVER diluted or "cut" in any way.