What is Knight in Shining Armor?

In the opera Julie goes to see in France, a princess is waiting on her knight in shining armor. Thomas represents a knight in shining armor to Julie. First, his business connections can offer Julie more insight into her dating research. Secondly, he is handsome and charming. Third, he escorts her around Paris. Ironically "armor" and "amour" are very similar words. Amour suggests a secret love affair, which Thomas suggests to Julie.