bathtub curve

noun

mathematics

: a curve with large initial and final values and a low, flat region in the middle

The trend of this cracking over an airplane's life follows what is known as the "bathtub" curve, so-called because its shape resembles that of a bathtub. The line of the curve charts the rise and fall in the number of cracks a typical fleet of aircraft experiences over its lifetime.—

Oswald Johnston, Los Angeles Times, 1 May 1988

Note: The bathtub curve is often used to describe the rate of failure of a product such as an electronic device or a piece of furniture. Such products often have a high failure rate in the beginning of their lifespan (as because of manufacturing defects) and at the end of their lifespan (as they wear out and break down).