The "Great Powers" is an old-fashioned expression that mostly referred to Prussia, Russia, Britain, France and Austria, which dominated Europe during the 1800s.
Nowadays, the most powerful nations are more often referred to as "the superpowers." This term came into use after the Second World War. At that time, it would have included the US, the UK, and the Soviet Union. There's no objective definition, it's all opinion, but very roughly one might say that the "superpowers" are the permanent members of the United Nations Security Council: China, France, Russia, the UK, and the US.
Very off-topic, but I remember reading a book in which a group of children play a game of making "the sound of a sneeze" by saying "Russia," "Prussia," and "Austria" together, all at the same time.