What does the acronym MOC stand for?
MOC stands for "My Own Creation". 


  MOC 
  
  My Own Creation. Any LEGO creation designed and built by a LEGO fan without instructions. Generally pronounced “mock.”*


Found at this Lego glossary.

Brickipedia's version from their own glossary:


  MOC - My Own Creation, a fan- made LEGO model


*A side note: There may be some LEGO fans who pronounce "MOC" the way the individual letters are pronounced - effectively saying "Em-Oh-See".