The time it takes for a change of input to result in a change of output. In a system with significant delay, users might not understand the correlation between input and output.
Often times if there is a delay, users overcompensate. For example, if it takes 30 minutes between putting food in one’s mouth and a feeling of satiation, one could easily overeat. If it takes three minutes between adjusting the temperature of the water and the shower to reflect the new temperature, bathers might go through cycles of getting the shower too hot, and then too cold.
Other times, especially if the input is expensive, users might give up. If spending money on advertising doesn’t result in immediate increased traffic, a business might give up, despite the eventual gain realized.