A marquis was originally the title used for the lord who ruled the Marches or similar frontier lands. It eventually became a rank between dukes and counts. In England, a marquis was between a duke and an earl, or the eldest son of a duke. It was first used in England in 1385.
A marchioness is the female equivalent of a marquis. It is a title used by the wife of a marquis, the eldest daughter of a duke or lady holding the rank in her own right.
Marquis in the SCA
There is no rank that is equivalent to marquis in the SCA. No sad marquis for them!