A somewhat literary or old-fashioned term for a love letter; in modern casual speech ラブレター is more common.
See also: ラブレター
She keeps old love letters in her desk drawer.
恋文を書くのは初めてで、何を書けばいいかわからない。
It's my first time writing a love letter, and I don't know what to write.
I wrote her a love letter.
After school, I gave him a love letter.
I once received a love letter from a senpai.