The term parametric means that the 3D CAD software is based around parameters e.g. dimensions. In 3D CAD systems the dimensions drive the geometry. If the dimensions drive the geometry, when you change a dimension it causes the model size to change along with the relations used to create the features in the part which are also captured in the model. Using parametric modelling, the designer roughly sketches initial shapes and applies drawing dimensions and constraints to create models that have intelligence. Traditional CAD packages do not rely on the dimensions to drive the geometry. Parametric modellers store dimensions of the model, along with the models itself. If any of the dimensions is changed, the model re-builds itself using the new dimensions. The model as a result is highly intelligent and can be easily edited, updated and changed as needed.Restricting how the model will change when dimensions or relations are altered is known as "Design Intent". 

 
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