Sir
A title reserved in the SCA for members of the Order of the Chivalry who have taken knighthood as their chosen path. They are refered to as peers of the realm.
Since 'Sir' holds no distinction in modern American culture, it's a frequent stumbling block for newcomers to not call a gentleman they meet 'Sir'. Instead "m'Lord" is a title which can be used for gentlemen of any rank within the Society.
Note that some (but not all!) female knights prefer to use the more feminine title of Dame instead of Sir. If you are unsure of the Knight's preference, a "M'Lady Knight" works well.