What is the name of the economic concept that describes the value of the second-best item or activity that is given up when the best item or activity is chosen?

Opportunity cost.

Explanation:

Opportunity cost is defined as the cost incurred when and individual carries out an activity and not another one. It is the value of the second best item that is given up.

For example if a person has a choice of going to the movies instead of working. The monetary cost of going to the movie is the amount he will pay for the tickets, however economists also consider the cost of the foregone alternative that is working and earning a wage.

So the opportunity cost is the wage that would have been earned if he had gone to work.