Love felt by one person that is not reciprocated; a one-sided or unrequited crush.
彼に片思いしているが、まだ気持ちを伝えられない。
I have a crush on him, but I still can't tell him how I feel.
高校の頃の片思いの相手に、今でも時々会いたくなる。
I still sometimes want to see the person I had a crush on in high school.
片思いの切なさを歌にして人気になったシンガーがいる。
There is a singer who became popular by turning the pain of unrequited love into songs.
片思いが実って、二人は付き合い始めた。
The one-sided love bore fruit, and the two started dating.