What do we call a cloud of matter that has not yet formed into a star?

The term for a cloud of matter that has not yet formed into a star is a nebula. A nebula is a vast region in space filled with gas and dust, which can serve as the building blocks for new stars. In the process of star formation, regions within the nebula can collapse under their own gravity, eventually leading to the birth of stars.

While a solar nebula specifically refers to the rotating disk of gas and dust from which a solar system can form, the broader classification of nebula encompasses a variety of forms, including those that might eventually lead to star formation, but are not limited to the solar system alone. Additionally, comets are small celestial objects composed of ice and dust that form tails when they approach the sun, and stellar clusters refer to groups of stars that have formed together and are gravitationally bound. These definitions clarify why "nebula" is the most appropriate term for a cloud of matter in the early stages of star formation.