质 is a polysemous noun with three common meanings: 1) the inherent quality or nature of something (本质); 2) matter or substance (物质); 3) a pledge or hostage (抵押品/人质). The specific sense depends on context; in modern Chinese it often appears as a bound morpheme in compounds like 质量 (quality) or 物质 (material). As a standalone noun it is relatively formal and used in specialized contexts such as philosophy, materials science, and legal terms.