Fast Fourier Transformation.
FFT was invented in 1805, and it was applied here and there in the decades after it, but it wasn’t particularly widespread. However, in the age of computers FFT is used in audio editing, signal processing, scientific instruments, image processing etc. just to name a few examples. Currently, it’s practically everywhere.
FFT is a really complicated topic, so I won’t bore you with the details. If you want to read more, check the wikipedia article about it.