The thick disk is the Milky Way component whose stars are generally older and more metal-poor than those of the thin disk. In astronomy, “metals” means every element heavier than hydrogen and helium, so metal-poor does not mean the stars are made of metal; it means they contain fewer heavy elements.
The thick disk extends farther above and below the Galaxy’s main plane than the thin disk. Its stars also tend to have different motions, orbiting with greater vertical movement and somewhat slower rotational motion around the Galactic Center. These properties distinguish it from both the thin disk and the roughly spheroidal halo.
The thin disk contains much of the Milky Way’s ongoing star formation, including the Sun, while the thick disk formed earlier, when the Galaxy had undergone less chemical enrichment. A common mix-up is to choose the dark halo: dark matter surrounds the Galaxy, but it is not the stellar disk population described here.