la Nochevieja - New Year's Eve
  malhablado - foul-mouthed
  el museo - museum, gallery
  mundial - world (as an adjective), worldwide
  las Montañas Rocosas - Rocky Mountains
  la mora - blackberry, mulberry
  el modus vivendi - way of life
  el modista, la modista - fashion designer, clothing designer,...
  la madrastra - stepmother
  el maíz - corn (American English), maize...
  la mortandad - mortality, number of mortalities,...
  la mostaza - mustard
  el murciélago - bat (birdlike mammal)
  el mote - nickname
  la moto - motorcycle, motorbike
  el mirador - scenic viewpoint; enclosed, windowed...
  Marruecos - Morocco
  la memoria - memory, remembrance, academic paper,...
  la miel - honey, molasses, sweetness
  el muslo - thigh, drumstick (of a bird)