The wife of an
earl (in
England) or a
count (in
France or other countries using French titles) or a
woman who inherited the
rank from a
parent. Presumably there was once some
Anglo-Saxon female equivalent to "earl", but the
French word "countess" displaced it.
In Geoffrey Hughes' Swearing: A Social History of Foul Language, Oaths, and Profanity in English, he notes that "it is a likely speculation that the Norman French title "Count" was abandoned in England in favor of the Germanic "Earl" ...precisely because of the uncomfortable phonetic proximity to cunt, which in Middle English could be spelt counte." Apparently for "countess" and "viscount" the extra syllables kept the similarity from being a problem, so those French titles were kept in England.