Sintering or frittage is the process of compacting and forming a solid mass of material by heat[1] or pressure[2] without melting it to the point of liquefaction.

Heat and compaction fuse small particles into a dense bulk
Clinker nodules produced by sintering

Sintering happens naturally in mineral deposits or as part of a manufacturing process used with metals, ceramics, plastics, and other materials. The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum. The study of sintering in metallurgy powder-related processes is known as powder metallurgy. An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice. Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together.

The word "sinter" comes from the Middle High German sinter, a cognate of English "cinder".