How did Aristotle define a syllogism?

Aristotle defined the syllogism as a central method of inference in which, when certain premises are stated, something other than what is stated follows of necessity from their being so.12

In his work the Prior Analytics, Aristotle established the syllogism as a form of term logic consisting of three propositions: two premises and a conclusion. He identified three distinct roles for the terms within these propositions:12
* Major term: The predicate of the conclusion.1
* Minor term: The subject of the conclusion.1
* Middle term: The term that appears in both premises, serving to link the major and minor terms.13

Aristotle was the first to use schematic letters (variables) to represent these terms, allowing him to analyze the logical form of arguments independently of their specific content. He sought to determine which patterns of these terms—known as moods—constituted valid inferences, meaning the form of the argument ensures that true premises can never lead to a false conclusion.123