What term refers to the quality of being indifferent or lacking concern?

The term "insouciance" captures the quality of being indifferent or lacking concern particularly well. It conveys a sense of carefree nonchalance where someone exhibits an unconcerned or nonchalant attitude towards important matters or situations. This word is often used to describe a relaxed approach to life, where worries and responsibilities are either not acknowledged or not regarded with seriousness.

While "apathetic" also highlights a lack of concern or interest, it leans more towards an emotional disengagement often experienced in response to situations that might normally evoke a stronger response. "Stolid" refers to being calm, dependable, and showing little emotion or animation, focusing more on the ability to remain unemotional. "Detachment," on the other hand, indicates a sense of separation from emotional involvement, suggesting a more deliberate or conscious choice to be uninvolved.

In summary, "insouciance" denotes a relaxed, almost casual disregard that best fits the definition of indifference or lack of concern in a carefree and easygoing manner.