Why is Pulse Code Modulation called 'modulation' and not 'conversion'?
From Wikipedia's article on modulation:


  In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.


So all we need to do is identify a periodic waveform and a way to modify its properties by a signal.  One easy way to think of this:  the "carrier wave" is a dirac comb that pulses at the pulse repetition rate.  This signal is modified by convolving it with a pulse whose width varies as a function of the input.  This creates a pulse train.