In-compactness refers to something that is lacking compactness or density, thereby being spacious or spread out. The antonyms of in-compactness, therefore, would be compactness, density, and solidity. Compactness encompasses tight and unified structures, with little to no spaces in between. It is often used to describe objects or materials that are firmly packed or closely packed together, forming a mass that is difficult to break apart. Density, on the other hand, refers to how tightly packed a substance or material is, while solidity refers to something that is physically strong, stable or hard to break. These antonyms of in-compactness are essential in describing the opposite characteristics of objects, materials, and other physical properties.