What is the primary characteristic of essential oils when taken orally?

The primary characteristic of essential oils when taken orally is that they are volatile. Essential oils are composed of small, aromatic molecules that easily evaporate at room temperature, which is what makes them "volatile." This volatility allows essential oils to quickly release their fragrance and therapeutic properties. When ingested, their volatility plays a significant role in their absorption and effects on the body.

The other options do not accurately characterize essential oils when taken orally. While essential oils can be active and may have therapeutic benefits, their defining characteristic is their ability to evaporate. Stability can vary among different oils, but volatility is a consistent trait. Therefore, the understanding of essential oils' volatility is crucial in the context of their use in oral applications.