The Historical Knight
In Medieval times, a knight was a mounted warrior in the service of a Lord. The word 'knight' is derived from the German word 'knecht', which means 'servant'.
See also: Chivalry
Knights in the SCA
In the SCA, a knight is a fighter who has been elevated by the Crown to the Order of Chivalry for showing great prowess at heavy fighting (heavily armoured hand to hand fighting) as well as behaving in a chivalrous manner.
Knights are made by the current king, to whom they swear fealty. They are recommended by a council of the other knights, but the final decision to elevate a person to the Order is the prerogative of the Crown. Those who do not wish to swear fealty may become masters of arms.
Order of Chivalry
The Order of Chivalry is a peerage order of knights and masters of arms. They consult the crown on matters of chivalry and recommend new members for their order.