What is the difference between "mixdown" and "mixing"?

One's a noun, the other's a verb...

Mixing (the verb) is what you do to achieve a mixdown (noun). In Audition terms, the basic mixing process is to default all tracks and the master output to 0dB settings, so unless you open the mixer and alter anything, that's effectively hidden your mixing. But you've still done it - you've combined the results of more than one track to create a 'mixed' output. If you use any of the automation, or alter any settings at all in the tracks of your mix, then you are 'mixing' (actively, rather than passively). And it's not just levels; adjusting the EQ and adding effects is also a part of the process.

The mixdown is what you achieve when you run the whole process and create a single file of the results. When you export, then it can sometimes be slightly different if you create what are known as 'stems' - individual sections of a complete mix - like brass, percussion, whatever for a final mix elsewhere. But it is also perfectly possible to export your mixdown as well.