# What is Hidden Layer? Definition & Technical Mechanics

> **Source:** [SPIDITS AI AI Glossary](https://spidits.com/ai-glossary)  
> **Category:** Neural Architectures  
> **Canonical Citation:** [https://spidits.com/ai-glossary/hidden-layer](https://spidits.com/ai-glossary/hidden-layer)

## Definition
A Hidden Layer is a layer of neurons located between the input layer and the output layer in an artificial neural network, responsible for extracting and learning abstract features from input data.

## Architectural Mechanics & Deep Dive
A hidden layer is an intermediate layer of neurons located between the input layer and the output layer in an artificial neural network. Hidden layers are where the network extracts and learns abstract, non-linear representations of input data. During training, as signal passes forward through hidden layers, the network learns increasingly complex features (e.g., from raw pixels, to edges, to faces), enabling the system to solve complex predictive tasks.

## Industry Applications & Real-World Use Cases
Feature extraction in multi-layer perceptrons, convolutional layers in computer vision, and feedforward networks in Transformers.

## Frequently Asked Questions

### Why are they called 'hidden' layers?
Because their inputs and outputs are not directly visible to the external environment, unlike the initial input layer and final prediction output layer.

### How does adding more hidden layers affect a neural network?
It increases the network's capacity to represent complex non-linear functions (making it 'deeper'), but also increases the risk of overfitting and training computation time.

## Related Terminology
- [neural-network](https://spidits.com/ai-glossary/neural-network)
- [fully-connected-layer](https://spidits.com/ai-glossary/fully-connected-layer)
- [deep-learning](https://spidits.com/ai-glossary/deep-learning)

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*From the SPIDITS AI Knowledge Base (336+ terms indexed). Explore full technical definitions: [https://spidits.com/ai-glossary](https://spidits.com/ai-glossary)*