activation

Activation is the process of making something active, functional, or operative after a period of dormancy or in response to a trigger. The term is used across sciences and technology to describe the transition from an inactive to an active state.

In chemistry and physics, activation refers most often to activation energy—the minimum energy required to start a chemical reaction. Reactions proceed when reactants acquire sufficient energy to overcome an energy barrier. Catalysts or enzymes lower this barrier, enabling the formation of an activated complex that leads to products.

In biology and medicine, activation describes the triggering of cellular responses. Immune cell activation, for example, occurs when receptors recognize a specific antigen and receive additional co-stimulatory signals. Activated lymphocytes proliferate and differentiate into effector cells. Activation also describes processes such as the activation of signaling pathways, transcription programs, and metabolic states.

In pharmacology, activation can refer to prodrugs that are administered in an inactive form and are metabolically converted into an active drug. This activation affects onset, duration, and distribution of the therapeutic effect.

In computer science and artificial intelligence, activation often refers to an activation function in neural networks. An activation function maps a neuron's input to its output, enabling nonlinearity and learning. Common examples include the sigmoid, tanh, and rectified linear unit functions.

In software and licensing, activation is the process of validating a product key or license with a vendor to unlock full functionality. Activation helps protect against unauthorized use and ensures compliance.