The problem with many text books concerning any subject is that each text book tends to define something in its own wording. One such definition states that opportunity cost is, "obtaining something (either a
service, a
product, or time, whatever
tangible or
intangible thing that can be obtained) by giving up something else." The best way to make any definition concrete is by stating examples, and listed below are some examples.
John Doe is faced with the choice of obtaining a note book from company A or company B, Doe chose company B, the cost of Doe's choice is giving up A to obtain B. Why? Because B is his best alternative, and there are many variables to consider, starting from price, his income, and the features he liked when comparing A over B.
John Doe is considering between working a full time job that pays $20,000, or getting a bachelor degree that will give him $40,000 after graduation. Doe choses education over work, the oppurtunity cost of education is losing the $20,000 job because education is his best alternative. However, he could work, and by working the oppurtunity cost of his working is losing the $40,000 income after graduation.
It all boils down to the best alternative since economics usualy have too many variables to take into consideration.