In Chariman Mao's "Three Worlds Theory" the Two Superpowers (the US and the USSR after capitalism was restored there in the 1950's) are the First World. The second world is the modernized secondary powers (Europe, etc.) The third world is the exploited nations (i.e. most of Africa.) So Belgium and Germany are Second World powers, the US and USSR are the First World.
This was in opposition to the classic cold war use of 'Three Worlds' in western foreign policy, which defined the US and allies as 'The First World', the USSR and allies as the Second World, and the rest as the Third World.
Your definition is the modern mish-mash defnition from after the decline of the Soviet Union, which comes closer to Mao's meaning, but is still significantly different.
And the existence of Mao's theory and the older Cold War definition trashes your 'Anybody who has ever defined the term' claim. I've shown there are at least three ways the term has been used.