What is ### ?

[jm (Jean-Michel Collard)]

Hello,

I know #(Slot) and ##(SlotSequence) but what is ### ?
Have a good day all,
Jean-Michel

[kubapod (Kuba Podkalicki)]

It is not a single thing:

 In[]:=  ### // FullForm
Out[]:= Times[Slot[1],SlotSequence[1]]