Hydrothermal synthesis is a method used to create materials, particularly crystals, by using high temperature and pressure in a water-based solution. This process occurs in a sealed container, known as an autoclave, where water acts as a solvent. The conditions allow for the dissolution of reactants and promote the growth of desired products, often resulting in high-purity and well-defined structures. This technique is commonly applied in the fields of materials science, chemistry, and nanotechnology. It is particularly useful for synthesizing complex compounds, such as zeolites and metal-organic frameworks, which have applications in catalysis, gas storage, and environmental remediation.