What is a Universal Construction in Category Theory?

From pg. 59 of Categories for the Working Mathematician:

Show that the construction of the polynomial ring $K[x]$ in an indeterminate $x$ over a commutative ring $K$ is a universal construction.

Question: What does the author mean by this bolded term?

For context: up to this point in the book, the author has already defined the notions of universal arrow, universal element, and universal property.

Is the author therefore just using the term universal construction as a synonym for the term universal property (or universal X, where X could be either element or arrow)?

A universal construction is simply a definition of an object as "the unique-up-to-isomorphism object satisfying a certain universal property". The name "construction" is a little misleading, because it's a definition: one still needs to show that the object actually exists, and that usually has to be done non-category-theoretically.

The term "universal construction" isn't that widely used in my experience, but it's pretty clearly a construction that is characterized by a universal property. In general, as Patrick Stevens and the nLab page states, you actually have to make a real construction to show that something satisfies the characterization. In some formal/logical/type theoretic contexts, you can view this as providing rules for completely syntactic objects. In effect, you are axiomatically assuming the existence of an object that satisfies the universal property. In this case though, the exercise is to find a universal property which characterizes the polynomial ring construction. A bit more colorfully, the exercise is to find the "essence" of the polynomial ring construction and present it in the form of a universal property.