API
Application Programming Interface; a software bridge that allows two different computer programs to talk to each other.
In Plain English
An API is like a waiter in a restaurant. You (the customer) look at the menu and tell the waiter what you want; the waiter takes the order to the kitchen (the AI system), and brings your food (the answer) back to you. APIs allow companies to build AI into their own apps without having to invent the AI themselves. For example, a travel app uses an API to send your questions to OpenAI and display the answers in their own app.
Real-World Example
A budgeting app using an AI API to automatically categorize your grocery store receipts.