Theano is a powerful Python library designed for efficient numerical computation, particularly in the realm of deep learning. Developed by the LISA group at the University of Montreal, it serves as a compiler for mathematical expressions, enabling developers to execute code on both CPUs and GPUs. Theano simplifies the creation of deep learning models by allowing users to define symbolic expressions, which can be compiled and optimized for performance. Although it is no longer actively maintained, Theano remains a foundational tool in machine learning and data science, often used in conjunction with higher-level libraries like Keras and Lasagne for enhanced usability.