What is the term for a psychological impairment that significantly limits one's ability to function?

The term that most accurately describes a psychological impairment that significantly limits one's ability to function is "mental disability." This term is widely used in legal and psychological contexts to refer to a condition that substantially restricts an individual's major life activities due to a mental or emotional condition. In legal settings, mental disabilities are important for considerations relating to rights, accommodations, and eligibility for support services.

The other terms, while related, do not capture the full legal and functional implications of the concept. "Mental disorder" typically refers to a diagnosis made within the framework of mental health conditions but does not inherently indicate the level of functional impairment. "Psychological dysfunction" generally describes issues in mental processes or behavior but lacks the specificity of significantly limiting functional capability. "Behavioral impairment" focuses more on observable actions rather than the underlying psychological conditions impacting overall functioning. Thus, "mental disability" encompasses the essence of the question with respect to functional limitations.