Which term refers to an unduly high opinion of oneself or one's abilities?

The term that refers to an unduly high opinion of oneself or one's abilities is "conceit." It specifically denotes an inflated self-image or a portrayal of oneself that is often seen as arrogant or vain. Conceit implies that an individual believes they are superior in some way, whether in skills, achievements, or qualities, which can lead to a lack of humility and an overestimation of one's own worth.

In contrast, "concord" refers to agreement or harmony between people or groups, which does not relate to personal assessment of abilities. "Conjoin" means to join together or unite, and is more about combining elements rather than assessing self-worth. "Congeal" describes the process of becoming solid or thick, typically in relation to substances like liquids, and is not relevant to personal opinion or perception. Therefore, among the given choices, “conceit” is the only term that accurately captures the essence of having an inflated view of oneself.