What is an opportunity cost?
Keat is mistaken. Most economists agree with Mankiw. The term oppportunity cost includes all costs, including explicit out-of-pocket ones and any other implicit ones.
As Henderson states:

The word “opportunity” in “opportunity cost” is actually redundant. The cost of using something is already the value of the highest-valued alternative use. But as contract lawyers and airplane pilots know, redundancy can be a virtue. In this case, its virtue is to remind us that the cost of using a resource arises from the value of what it could be used for instead.

Here are some definitions from introductory economics textbooks:

The next-best good that is forgone represents the opportunity cost of a decision. (Samuelson &amp; Nordhaus, Economics, 2010, p. 13)
Opportunity cost is the benefit that you might have gained from choosing the next-best alternative. (Colander, Microeconomics, 2017, p. 9)
We refer to this best alternative activity as the opportunity cost. (Acemoğlu, Laibson, &amp; List, Microeconomics, 2016, p. 41)
Economists call that kind of cost—what you must give up in order to get an item you want—the opportunity cost of that item. ... The opportunity cost of an item—what you must give up in order to get it—is its true cost. (Krugman &amp; Wells, Economics, 2015, p. 7).

Of course, as mentioned by Henry, you can define any term as you please. So the question might be, &quot;What is the standard or most commonly used definition among economists?&quot; And the answer is, &quot;Mankiw's and not Keat's.&quot;