
A neural network with multiple hidden layers, capable of learning complex patterns from data.A deep neural network is an artificial intelligence model composed of multiple layers of interconnected nodes (neurons), each layer transforming the input data into increasingly abstract representations. The 'deep' in the name refers to the many hidden layers between input and output, which allow the network to learn hierarchical features — from simple edges in images to complex objects, or from phonemes to full sentences in speech.
The concept of deep architectures dates back to the 1960s with the perceptron, but training deep networks was impractical until the 2000s when techniques like layer-wise pre-training and ReLU activations, along with GPU computing, made deep learning feasible. The 2012 AlexNet victory in ImageNet marked the modern deep learning revolution.
Each neuron in a layer computes a weighted sum of its inputs, adds a bias, and applies a nonlinear activation function (e.g., ReLU). During training, the network adjusts these weights using backpropagation, which calculates the gradient of a loss function with respect to each weight and updates them via gradient descent. With enough data and computational power, the network learns to minimize prediction errors. The hierarchical structure means early layers capture low-level features, while deeper layers combine them into high-level concepts.
Deep neural networks are used in image recognition, natural language processing, autonomous driving, medical diagnosis, and scientific research — from detecting gravitational waves to classifying galaxies.
Deep neural networks can be fooled by adding tiny, imperceptible perturbations to images, causing them to misclassify a panda as a gibbon with high confidence — a phenomenon called adversarial examples.









