The Maillard reaction is a group of chemical reactions between reducing sugars and amino compounds that produces brown colors and many aromas associated with cooked food. It contributes to the crust of bread, the flavor of roasted coffee, and the browned surface of seared meat.
Heat speeds these reactions, while time, acidity, and moisture also affect them. A relatively dry surface browns more readily during searing than a wet one. There is no single temperature threshold that describes every food and process.
Maillard browning is different from caramelization, which involves the breakdown of sugars under heat without requiring amino compounds. Both processes can contribute to the color and flavor of a food.
In the kitchen, drying the surface of ingredients and avoiding an overcrowded pan can help browning. Browning is a flavor-development process, not a reliable test of whether food has reached a safe internal temperature.