Theano is an open-source numerical computation library that allows users to define, optimize, and evaluate mathematical expressions involving multi-dimensional arrays. It was developed by the Montreal Institute for Learning Algorithms and is particularly useful for deep learning applications. Theano enables efficient computation on both CPUs and GPUs, making it a popular choice for researchers and developers in the field of machine learning. Although Theano was widely used in the past, its development has slowed down, and it is no longer actively maintained. Many users have since transitioned to other libraries, such as TensorFlow and PyTorch, which offer more features and community support for modern machine learning tasks.