What do we call a group of stars that form an imaginary picture in the sky?

A group of stars that form an imaginary picture in the sky is called a constellation. Constellations are patterns recognized throughout history, often based on mythology or cultural stories. They help astronomers and stargazers locate stars and navigate the night sky.

In contrast, a galaxy refers to a massive system that contains billions of stars, along with gas, dust, and dark matter, all bound together by gravity. A cluster is a smaller grouping of stars or galaxies, typically held together by gravitational forces, while a solar system consists of a star and all the celestial bodies that orbit it, such as planets, moons, asteroids, and comets.

Thus, the term that specifically describes the imaginary shapes formed by stars in our sky is indeed constellation.