Dictan - Dictionary Viewer

Dictan is a free Dictionary Viewer for Android which supports .fdb and .zd/.zpak dictionary bases. These formats are open, so you may download lots of free .zd dictionaries, handbooks, and encyclopedias from the Internet or create your own ones. Put .zd bases into /sdcard/dictan/ and they are ready for viewing! All extra info and the tools to create your own dictionaries are available at WWW.SOFTEX.INFO

The web-site also has the following free dictionaries in the 'Downloads' section (plus many others available over the section of 'ZD resources'):

English Idioms
English Abbreviations
English Synonyms
Wiki
Other various dictionaries

Recent changes:
New: Supported search in all dictionary bases (Beta version)
New: Minimum version of Android API is increased to 19 (4.4 KitKat)
New: Target version of Android API is increased to 28 (9.0 Pie)
New: Removed support of article text view
New: Added Spanish and German interface languages
Improved: Changed buttons in main application activity
Improved: Added icons in file navigator and ability to enter path manually
Fixed: Many bugs related to specifics of Android versions