The "lean startup" is a business methodology that emphasizes rapid product development and customer feedback. It encourages entrepreneurs to create a minimum viable product (MVP) to test their ideas quickly in the market. By gathering data from real users, startups can make informed decisions about their product and pivot if necessary. This approach reduces waste and increases the chances of success by focusing on what customers actually want. The lean startup method was popularized by Eric Ries in his book, which outlines principles like validated learning and iterative design to help startups grow efficiently and effectively.