GDP is the grand total of everything a country produces and sells in a year. Think about every transaction in a single country over twelve months: every coffee sold at a campus café, every phone assembled in a factory, every haircut given, every bridge built. Add up the final price tags on all of those — not the raw materials, just the finished things people actually buy — and you have GDP.

More precisely, GDP is the market value of all final goods and services produced within a country's borders during a specific period, typically a quarter or a year. The word final is doing real work here: it excludes intermediate goods — the steel that goes into a car is not counted separately, because its value is already captured in the car's price. Counting it twice would overstate output, a problem called double-counting. Within borders is what separates GDP from GNP (Gross National Product): GDP counts what is produced on a country's soil regardless of who owns the factory; GNP counts what is produced by a country's residents regardless of where they work.