GD&T, or Geometric Dimensioning and Tolerancing, is a system used in engineering and manufacturing to define the allowable variations in the size and shape of parts. It provides a clear and concise way to communicate how parts should be manufactured and inspected, ensuring they fit together correctly in assemblies. This system uses symbols and annotations on technical drawings to specify features like flatness, cylindricity, and position. By using GD&T, engineers can improve product quality, reduce manufacturing costs, and enhance communication between design and production teams.