Eigen is a popular open-source C++ template library designed for linear algebra, matrix, and vector operations. It provides efficient and easy-to-use tools for mathematical computations, making it a favorite among developers and researchers in fields like machine learning, computer graphics, and scientific computing. The library supports various matrix types, including dense and sparse matrices, and offers functionalities for solving systems of equations, eigenvalue problems, and more. Its design emphasizes performance and flexibility, allowing users to integrate it seamlessly into their projects while benefiting from optimized algorithms and data structures.