countess
The wife of a count, or a woman holding the rank of count in her own right. Used in historical or aristocratic contexts.
The countess was devoted to charity work.
伯爵 is the title 'count' (male or gender-neutral), while 伯爵夫人 specifically refers to a countess.
Compound of 伯爵 (count) and 夫人 (wife, lady). The term follows the pattern of European noble titles adapted into Japanese.