In Computer-Aided Design (CAD), a constraint is a rule or condition applied to a model to control its geometry, such as dimensions, relationships, or limitations. Constraints are used to define and maintain the desired shape, size, and behavior of a model, ensuring that it meets the required design intent and functionality.

There are several types of constraints commonly used in CAD, including:

1. Dimensional Constraints: These constraints specify the size or distance between features in a model, such as the length of a line, the diameter of a circle, or the distance between two points.

2. Geometric Constraints: These constraints define the relationships between features in a model, such as parallelism, perpendicularity, tangency, or concentricity. Geometric constraints ensure that features maintain their relative position and orientation.

3. Assembly Constraints (also known as Mates): These constraints define the relationships between parts in an assembly, such as aligning faces, edges, or points, or defining the distance or angle between components.

4. Motion Constraints: These constraints define the allowed motion or degrees of freedom of parts in a mechanism, such as limiting a part to rotate around a specific axis or translate along a certain path.

Constraints are a fundamental concept in parametric modeling, where the geometry of a model is defined by parameters and relationships. By applying constraints to a model, designers can create dynamic, flexible designs that can be easily modified and updated by changing the values of the parameters that define them.

Constraints also play a crucial role in ensuring the integrity and consistency of a model. By defining the relationships and limitations between features, constraints help to prevent errors and inconsistencies in the model, such as overlapping or disconnected geometry.

Most CAD software packages provide a range of tools and features for applying and managing constraints, including automatic constraint detection, constraint libraries, and constraint editing tools. These tools allow designers to quickly and easily apply constraints to their models, and to modify and update constraints as needed throughout the design process.