non-volatile means a liquid that won't evaporate at all. It's not a commomly encountered term because there is usually some evaporation (especially as heat is applied). The usual phrase is "very low volatility". Oil at room temperature or colder fits this category.

How can I define non-volatile? (Besides saying it's the opposite of volatile)

Thanks

To define non-volatile, you can describe it as a property of a substance or liquid that refers to its resistance to evaporating or vaporizing easily at typical conditions. Unlike volatile substances, which readily evaporate or boil at relatively low temperatures, non-volatile substances show very little evaporation or require significantly higher temperatures to vaporize.

Apart from describing it as the opposite of volatile, another way to define non-volatile is by explaining its behavior in terms of vapor pressure. Vapor pressure is the pressure exerted by the vapor (or gas phase) above a liquid or solid when the substance is in equilibrium with its surroundings. Non-volatile substances typically have low vapor pressures, meaning that the amount of vapor they produce is minimal under normal conditions.

Additionally, it's worth mentioning that non-volatile substances can often be referred to as having "high boiling points" or "very low volatility." These terms further emphasize their resistance to evaporation. For instance, oils that remain in liquid form at room temperature or colder are considered non-volatile since their low volatility prevents them from easily evaporating.