Definition of curtsy:
1. [n] bending at the knees; a gesture of respect made by women
2. [v] make a curtsy; usually done only by girls and women; as a sign of respect; “She curtsied when she shook the Queen’s hand”
3. [v] a gesture of respectful greeting, for women
Invitation to Dance, by Alan Maley
When a girl or woman curtsies, she bends quickly at the knees, with one foot in front of the other, while holding her skirt, especially to show respect to a king or queen.
Trivia:
One of Wimbledon’s most enduring traditions is finished – players will no longer have to bow or curtsy to the Royal Box at Centre Court.
…
Players have been required to bow or curtsy to the royal family when walking onto or leaving Centre Court. From now on they will have to do so only if Queen Elizabeth II or Prince Charles, her eldest son and heir to the throne, is in the box.
With curtsy to each other / went full many a maid,
The knights a willing service / unto the ladies paid.
After the greeting sat they / adown upon the green;
Knew many then each other / that hitherto had strangers been.Excerpt from Chapter 24, The Nibelungenlied (By George Henry Needler, Translator)
The actual ceremony was quite simple. The young woman would have her name announced by the Lord Chamberlain, she would walk forward to where the monarch was seated and make a deep curtsy. She would then curtsy to any other royal personages and then back out of the royal presence, all the while trying not to drip over her train.
The young debutante would also practice kissing the Queen’s hand, but the curtsy was all-important! Often a young lady attended classes to learn to curtsy in the proper
manner. The curtsy she performed at Court was not an ordinary curtsy. This was a full court curtsy; one where she would need to bend her knee until it nearly touched the floor–but not quite.Then she would hold this position for an ample amount of time while making a low bow, and rise again, without losing her balance, falling over, or tripping on her gown and its extensive train. Finally, the young lady repeatedly practiced her exit because she would be required to back out of the room, as it was considered the height of impropriety and was against all rules of etiquette to ever turn her back on a royal personage.
“Stop,” she said, “I am grown too big! Welcome, cousin Harry,” and she made him an arch curtsy, sweeping down to the ground almost, with the most gracious bend, looking up the while with the brightest eyes and sweetest smile. Love seemed to radiate from her. Harry eyed her with such a rapture as the first lover is described as having by Milton.
February 13th, 2005 | 
