What is the definition of mercurial?

The definition of "mercurial" refers to something that is quick and changeable in temperament. This term is often used to describe people who exhibit rapid shifts in mood or behavior, similar to the nature of the planet Mercury, which is known for its speed and variability. Individuals who are described as mercurial might have emotional responses that can change swiftly, making it an apt descriptor for those who are unpredictable or volatile in their feelings or reactions.

The other options illustrate traits that stand in contrast to the concept of being mercurial. Describing someone as steady and calm, predictable and stable, or reserved and quiet suggests a lack of fluctuation in mood or temperament, which is opposite to the essence of being mercurial. Therefore, the choice that captures the dynamic and variable nature of a person's temperament is indeed the one that defines "mercurial" accurately.