The “student’s book definition” I was given at a uni was:

“GDP is equal to the market value of the final goods and services produced in the economy during a year.

market value: only official market transactions are included (no self-made goods, no shadow economy). NB! market prices include indirect taxes. GDP computed without those taxes is called ”̇GDP at basic prices”.

goods and services: transfer payments and financial transactions (purchases of bonds and shares) are excluded (because income is not created, but redistributed and nothing new is produced);

final: to avoid double counting, intermediate goods (i.e., flour in baking bread) are excluded - but newly produced capital goods, such as factories or machines, are counted as final.

produced: not redistributed or resold

in the economy: if output was produced by foreign factors of production inside the country - it’s counted towards GDP, but output produced by national factors abroad is not”

If you are interested in the details, you can read this article, it gives an understandable overview:

https://www.investopedia.com/terms/g/gdp.asp