Final answer:
The phase described in the question is the Death Phase, characterized by a declining population growth rate and an exponential decrease in the number of cells in a culture medium.
Step-by-step explanation:
The phase described in the question is known as the Death Phase. In this phase, as a culture medium accumulates toxic waste and nutrients are exhausted, the number of dying cells exceeds the number of dividing cells, leading to an exponential decrease in the number of cells. This phase is characterized by a slowing and eventual halt in population growth, as the total population of living cells becomes relatively stagnant.