Which term refers to a state of being skeptical or disbelieving?

The term that refers to a state of being skeptical or disbelieving is "incredulous." When someone is described as incredulous, it means that they are unable or unwilling to accept something as true, often due to a lack of belief or a strong sense of doubt regarding the information presented to them. This aligns perfectly with the context of skepticism or disbelief.

In contrast, the other terms do not carry this meaning. "Incessant" refers to something that continues without interruption, highlighting a continuous nature rather than a state of disbelief. "Indifferent" describes a lack of interest, concern, or sympathy, which does not directly relate to skepticism. "Ineffable" refers to something that is too great or extreme to be expressed in words, which also does not convey a sense of disbelief or skepticism. Thus, "incredulous" is the most appropriate term for describing a state of skepticism.