What makes something a syllogism?

A syllogism consists of a major premise, a minor premise, and a conclusion. It is a formal scheme of a logical argument. Syllogisms can result in true conclusions correctly drawn, or they might lead to absurd or subtlely wrong conclusions for several different reasons that are part of the study of logic. For example:

Light is the visible range of the electromagnetic spectrum.
Jane's lunch was light.
Therefore, Jane's lunch was the visible range of the electromagnetic spectrum.

Or,

Each element has a unique atomic number.
Carbon is an element.
Therefore, carbon has a unique atomic number.