Cost-per-impression (CPI) is a marketing metric that calculates the cost associated with each time an advertisement is displayed to a viewer, regardless of whether or not the viewer interacts with it. This metric is crucial in determining the effectiveness of advertising campaigns and assessing return on investment, as it helps organizations understand how much they are spending to reach their target audience. By analyzing CPI, public relations professionals can evaluate the financial efficiency of their media placements and make informed decisions about budget allocations.