By definition:

a thick sweet liquid made by dissolving sugar in boiling water, often used for preserving fruit.

In cooking, syrup is a condiment that is a thick, viscous liquid consisting primarily of a solution of sugar in water, containing a large amount of dissolved sugars but showing little tendency to deposit crystals. In its concentrated form, its consistency is similar to that of molasses.

The "syrup" in pop historically refers to the liquid sugar component of it, where as in modern times it's a bit more colloquial where with substitutes and whatnot it's still "syrup" in industry parlance but isn't strictly a syrup.

Aspartame, flavouring, food colouring, water, co2 - the flavoring, colouring and aspartame does not strictly constitute a "syrup" simply because it's mixed together