Why is opportunity cost called a 'cost'?

Opportunity cost is the cost of the alternative opportunity given up. Individuals always face a choice among the different alternatives available. A producer, while producing a good can choose a combination with more labor or more of capital. If the production of good can increase by using more of both the resources but the producer wants only that combination of inputs which has more of labor only, then producer will bear a loss in production caused by using less amount of labor. Similarly, an individual has monotonic preferences, which mean more of each good is preferable. The individual likes both the goods X and Y and faces a choice of consumption between the two. If the consumer wants to consume more of one good X then due to the limited income, the consumption of good Y will decrease which imposes a cost on the consumer as consumption of good Y is forgone. So, this can be concluded that opportunity costs are considered as a cost.