Reconstruction filter
The low-pass filter after a DAC that removes the images above the Nyquist frequency and turns samples back into a smooth waveform.
A DAC outputs a staircase, and the steps carry copies of the audio spectrum around multiples of the sample rate. The sampling theorem promises the original can be recovered exactly, but only if everything above half the sample rate is filtered away. That is the reconstruction filter's job – the mirror image of the anti-aliasing filter on the recording side.
The first CD players in 1982 needed steep analogue filters between 20 and 22 kHz, which were expensive and twisted the phase in the treble. Philips chose from the start to oversample four times and do most of the filtering digitally, leaving a gentle analogue stage. That became the norm. Audiophile "non-oversampling" DACs amount to a stated preference for hearing a few images.