to hold a torch for someone (British)
to carry a torch for someone (American)

idiom

- to be in love with someone without them knowing it

- to be in love with someone who is no longer in love with you


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WORD ORIGIN
O! She doth teach the torches to burn bright
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear;
Beauty too rich for use, for earth too dear.

As this passage from Shakespeare's Romeo and Juliet shows, the torch has long been associated with passion and love. But even before the great English bard of Avon, the ancient Greeks and Romans used the torch in marriage ceremonies. Brides customarily carried a torch to light the hearth in their new home. Hymen, the god of marriage ceremony, is often represented in paintings as a youth holding a torch. And let's not forget Venus, the goddess of love, who is frequently depicted with a torch.

Apart from its reference to matters of the heart, the torch has taken on other meanings. The Olympic torch for instance, which was first used to light the Olympic flame at the 1936 Summer Olympics in Berlin, symbolises the transmission of Olympic ideals from ancient Greece to the modern world. And one of the most recognisable torches in the world can be found soaring above New York harbour, raised with one hand by the Statue of Liberty. It's meaning is open to interpretation, but the U.S. National Park Service, which maintains the 100-metre high copper statue, says:

"The torch is a symbol of enlightenment. The Statue of Liberty's torch lights the way to freedom, showing us the path to Liberty. Even the statue's official name represents her most important symbol: Liberty Enlightening the World.”

But back to love for a moment. Exactly how the torch came to symbolise romantic obsession is not really clear. The Word Detective cites one possible source as the eccentric Greek philosopher Diogenes (412-322 B.C.), who wandered around with a torch in a futile attempt to find an honest man. He apparently found only rascals and scoundrels. But we believe rather than one single source, like many words and expressions in our language, "holding a torch” simply evolved into its current context over a long period of time.

*False friend warning! Be careful if you travel to the United States and need to buy a torch. In both British and American English, a torch can refer to a portable light produced by a flame from a stick or other flammable material. However, a battery-operated light is called a torch in British English, but a flashlight in American English.

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SYNONYMS
There are no direct synonyms for the expression "to hold a torch”, but there are several nice words and phrases that simply describe being in love. Please note the preposition usage. It's key to making the phrase work:

to be smitten with, to be infatuated with, to be wild about, to be crazy about, to yearn for, to be hooked on, to be nuts about, to be stuck on, to be attracted to, to be fond of, to be taken with, to fancy

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IMPRESS YOUR FRIENDS TODAY
say something like:

"Tomorrow's meeting should be interesting. One of the people attending is a young lady that Todd has been holding a torch for since they were in college.”