A decreasing function is a type of mathematical function where, as the input value increases, the output value decreases. This means that for any two points on the function, if the first point has a smaller input than the second, the output of the first point will be greater than the output of the second. In simpler terms, if you were to plot a decreasing function on a graph, the line would slope downwards from left to right. Common examples of decreasing functions include linear functions with a negative slope and certain types of exponential functions that decline over time.