The Dictionary of Biology explains the meaning of words in Spanish, based in Spanish. Free!

Characteristics:
♦ Thousands of definitions.
♦ Offline! No internet connection needed. Internet connection is used only when a word is not in the dictionary
♦ You can browse through words with your finger!
♦ Voice Recognition
♦ Issues with color text user-defined
♦ Bookmarks and History
♦ random search button, useful for learning new words
♦ interface simple and functional user
♦ Copy to clipboard definitions
♦ dark Interface
♦ Share your favorite definitions