What phenomenon describes the decrease in the additional output generated by adding one more unit of a variable input while keeping other inputs constant in the short run?

The phenomenon you are asking about is called diminishing marginal returns. This occurs when you add more of one input, like workers, while keeping other inputs, such as machinery, the same. At first, adding more workers can lead to a significant increase in output because they can help each other and work more efficiently. However, after a certain point, each new worker contributes less and less to the total output because they have limited machinery to use, and they may get in each other's way. For example, if a factory has one machine and you keep adding workers, eventually, those extra workers will not be able to work effectively together, and the increase in production will slow down.