Definition & Meaning of "[carry] a torch for {sb/sth}"

What is the origin of the idiom "carry a torch for somebody or something" and when to use it?

The idiom "carry a torch for somebody or something" is believed to have originated in the 19th century. It is thought to be a reference to the fact that a torch was a symbol of love and devotion in ancient Greece. It is used to describe having strong and enduring feelings of love or affection for someone or something. Often used to describe having strong feelings of romantic love or infatuation for someone who may not feel the same way. It implies a one-sided affection or a persistent longing for someone who is often unaware of or uninterested in the person's feelings.