In chemistry, the term nonvolatile refers to a substance that does not readily evaporate into gas under existing conditions. In other words, a nonvolatile material exerts a low vapor pressure and has a slow rate of evaporation.
Examples
Glycerin (C3H8O3) is a nonvolatile liquid. Sugar (sucrose) and salt (sodium chloride) are nonvolatile solids.
It's probably easier to imagine a nonvolatile substance if you consider the properties of materials that are volatile. Examples include alcohol, mercury, gasoline, and perfume. Volatile substances readily release their molecules into the air. You typically don't smell nonvolatile materials because they don't easily convert from liquids or solids into gases.