Kernel
Aristotle is the figure in whom Western thought first treats reality as a structure that can be exhaustively categorized. The Lyceum's lectures cover physics, biology, ethics, politics, rhetoric, and the formal logic that becomes the operating syntax of Western reasoning for two thousand years.
Contribution
Formal logic (syllogism). The principle of non-contradiction. The empirical method in zoology. Four-cause analysis (material, formal, efficient, final). The Politics's classification of regimes. A corpus running to roughly a million words that frames every subsequent Western philosophical tradition either by extending it or rebelling against it.
Civilization-scale significance
The figure whose vocabulary the West argues with for the longest time. Most undergraduate philosophy is still organized around "things Aristotle said and things people said in response."