The practice of interviewing potential customers before you build anything, to confirm the problem is real and painful enough that someone would actually pay to fix it.
After running and advising eight startups, Steve Blank wrote The Four Steps to the Epiphany to argue that startups fail because they execute on assumptions instead of testing them. Customer discovery was step one: get out of the building, talk to potential users, and validate the problem before writing any code. The book became the foundation of the lean startup movement that Eric Ries later popularized.