A reference dimension is a measurement used in engineering and technical design to define the size or position of a feature or object. There are several synonymous terms used in engineering to refer to reference dimensions such as datum dimension, basic dimension, and key dimension. Datum dimension refers to the measurement that defines the location or orientation of a part or feature. Basic dimension is a dimension that does not include any tolerance, and it defines the nominal size of a part or feature. Key dimension is a term used to describe a dimension that is critical for the function or performance of a part or feature. The use of these interchangeable terms is important in ensuring accurate communication between designers and manufacturers.