What is the MEL (minimum equipment list)?
As mentioned here:


  A minimum equipment list is a list of equipment that must be installed
  and operable for the aircraft to be considered airworthy. It is
  aircraft-specific and spells out which pieces of equipment may be
  inoperable while maintaining airworthiness. If something is found to
  be inoperative, the pilot goes to the MEL, finds the entry for that
  item, and determines if the airplane must be grounded until that piece
  of equipment is fixed.


A Master minimum equipment list is:


  An MEL for a specific aircraft originates from a master minimum
  equipment list (MMEL). The MMEL is a list of all equipment on an
  aircraft type; it details which equipment is allowed to be inoperative
  without grounding the aircraft. (Think of a MMEL as a general, broader
  MEL.)


FAA publishes MMELs by manufactures.