Merriam Webster Mobile English Dictionary Series

Merriam-Webster is America's foremost publisher of language-related reference works ever. It published in a diverse print products available out there including in mobile dictionary version.
Several product that can be downloaded (demo version):

• Merriam-Webster’s Third New International Dictionary
• 
  Merriam-Webster's Collegiate Dictionary
• 
  Merriam-Webster's Pocket Dictionary
• 
  Merriam-Webster's Medical Dictionary
• 
  Merriam-Webster's Spanish-English Dictionary

Available in Windows mobile, Symbian, Android, Palm OS, iPhone, MS Windows
Download from here