Italicized portion of definition already given in glossary:

French AMOUR COURTOIS, in the later Middle Ages, a highly conventionalized code that prescribed the behaviour of ladies and their lovers. Amour courtois also provided the theme of an extensive courtly medieval literature that began with the troubadour poetry of Aquitaine and Provence in southern France toward the end of the 11th century. It constituted a revolution in thought and feeling, the effects of which are still apparent in Western culture. (click here for more)
 

The courtly lover existed to serve his lady. His love was invariably
adulterous, marriage at that time being usually the result of business
interest or the seal of a power alliance. Ultimately the lover saw
himself as serving the all-powerful god of love and worshipping his
lady-saint. Faithlessness was the mortal sin.

The philosophy found little precedent in other, older cultures.
Conditions in the castle civilization of 11th-century southern France,
however, were favourable to a change of attitude toward women.
Castles themselves housed many men, few women; poets, wishing
to idealize physical passion, looked beyond the marriage state. The
Roman poet Ovid undoubtedly provided inspiration in the
developing concept of courtly love. His Ars amatoria had pictured
a lover as the slave of passion--sighing, trembling, growing pale and
sleepless, even dying for love. The Ovidian lover's adoration was
calculated to win sensual rewards; the courtly lover, however, while
displaying the same outward signs of passion, was fired by respect
for his lady. This idealistic outlook may be explained partly by
contemporary religious devotions, both orthodox and heretical,
especially regarding the Virgin Mary, and partly by France's
exposure to Arab mystical philosophy (gained through contacts with
Islam during the Crusades), which embodied concepts of love--as a
delightful disease, as demanding of faithful service--that were to
characterize courtly love.

Courtly love may therefore be regarded as the complex product of
numerous factors--social, erotic, religious, and philosophical. The
idea spread swiftly across Europe, and a decisive influence in this
transmission was Eleanor of Aquitaine, wife first to Louis VII of
France and then to Henry II of England, who inspired some of the
best poetry of Bernard de Ventadour, among the last (12th century)
and finest of troubadour poets. Her daughter Marie of Champagne
encouraged the composition of Chrétien de Troyes's Lancelot (Le
Chevalier de la charrette), a courtly romance whose hero obeys
every imperious (and unreasonable) demand of the heroine. Soon
afterward the doctrine was "codified" in a three-book treatise by
André le Chapelain. In the 13th century a long allegorical poem, the
Roman de la rose, expressed the concept of a lover suspended
between happiness and despair.

Courtly love soon pervaded the literatures of Europe. The German
minnesinger lyrics and court epics such as Gottfried von Strassburg's
Tristan und Isolde (c. 1210) are evidence of its power. Italian
poetry embodied the courtly ideals as early as the 12th century, and
during the 14th century their essence was distilled in Francesco
Petrarch's sonnets to Laura. But perhaps more significantly, Dante
had earlier managed to fuse courtly love and mystical vision: his
Beatrice was, in life, his earthly inspiration; in La divina commedia
she became his spiritual guide to the mysteries of Paradise. Courtly
love was also a vital influential force on most medieval literature in
England, but there it came to be adopted as part of the courtship
ritual leading to marriage. This development, discussed in C.S.
Lewis' Allegory of Love (rev. ed., 1951), became more
pronounced in later romances.