In Star Trek: Deep Space Nine, a solar sail is the propulsion technology that powers the Bajoran lightship.
The lightship is featured in “Explorers,” when Benjamin Sisko and Jake Sisko reconstruct an ancient Bajoran vessel. Its large sails catch the pressure of radiation from a star, allowing the craft to move through space without warp engines or conventional impulse propulsion.
The episode uses the voyage to explore an old Bajoran claim that their people reached Cardassia centuries earlier. The ship's journey demonstrates that a solar-sail vessel could cross interstellar distances, although its trip would be extremely slow by modern Federation standards.
A solar sail is based on real physics: photons carry momentum, and a sufficiently large reflective sail can gradually accelerate a spacecraft. Unlike a warp drive, it does not create faster-than-light travel. The Star Trek vessel is a fictional extrapolation of this real concept.