Process using more memory than I physically have
This guy explains it well http://www.chriswrites.com/what-is-virtual-memory-on-the-mac/


  So What is Virtual Memory?
  
  Your Mac comes with a limited amount of
  physical memory (RAM) installed by default. Memory is critical to any
  computer to run applications and carry out all the operations
  commanded by the user. Memory must be as fast as possible (and it
  helps to have as much as possible) to maximise your Mac’s performance;
  thankfully, prices have drastically fallen in recent years.
  Nevertheless, every computer has a finite amount of memory so Mac OS X
  might actually need to use more than is physically available on your
  system.
  
  This is where virtual memory comes in – you can think of it as a kind
  of extension to the real memory on your system, albeit much slower
  because it is in fact physical space on your hard disk (or SSD, if you
  have one installed). Let’s say that you have 4 GB of main memory on
  your Mac and a 512 GB hard disk. If Mac OS X needs to use all 4 GB of
  main memory, it can still make use of any free space on your hard disk
  as an extension to that memory. Unfortunately, virtual memory is much
  slower than the physical RAM installed in your computer, because hard
  disks cannot match the speed of memory chips.