Meteor.js is an open-source full-stack JavaScript framework for building modern web and mobile applications. It allows developers to create responsive apps with built-in collaboration features that are fun and fast to write. Apps built with Meteor can hot code push, which reloads the app without losing user state, and be compiled for multiple platforms including web, iOS, and Android. The Meteor framework is based on 7 principles including data on the wire, one language, database everywhere, and latency compensation, which allow apps to feel responsive even on poor network connections.