API

Simple Definition

An API (Application Programming Interface) is a way for one piece of software to communicate with another. It’s a defined set of rules for making requests and receiving responses between systems.

In the AI context: when you build an app that uses ChatGPT or Claude, you connect to OpenAI’s or Anthropic’s API. Your app sends a request (a message), the AI processes it, and sends back a response.

A Simple Analogy

A restaurant menu is like an API. You don’t need to know how the kitchen works. You just order from the menu (make a request), and the kitchen returns food (a response). The menu defines what you can ask for and what you’ll receive.

How AI APIs Work

1. You sign up for an API key (your credential)
2. Your code sends an HTTP request to the API endpoint with your message
3. The AI processes your request
4. The API returns a response with the AI’s output
5. Your code uses that output in your application

Why AI APIs Matter

AI APIs let developers:

• Add AI capabilities to any application without hosting models themselves
• Access state-of-the-art models (GPT-5, Claude, Gemini) via simple calls
• Build products on top of AI without training models from scratch
• Scale AI features without managing AI infrastructure

Major AI APIs

• OpenAI API: access to GPT-5, image generation, Whisper
• Anthropic API: access to Claude models
• Google Gemini API: access to Gemini models
• Perplexity API: access to search-augmented AI
• Mistral API: access to open-weight models via API

API Pricing

Most AI APIs charge per token (input and output). Understanding this helps you estimate costs for building AI features.

Related Terms

• AI Integration, using APIs to connect AI to existing systems
• Function Calling, AI calling external APIs as part of tool use
• LLM, the AI models accessed via APIs
• No-Code AI, alternatives for accessing AI without using APIs directly