Collins Dictionary/Thesaurus (Free online) - define words and show like terms (based on US English).
Oxford Thesaurus/Dictionary (Free online) - define words and show similar words (based on UK English)
Merriam Webster Dictionary/Thesaurus (Free online) - define words and list synonyms (used in Canada as well)
Martin, E. A., & McFerran, T. A. (Eds.). (2014). A dictionary of nursing. Oxford University Press. (parts freely available through Google books).
Arabic Ontology & Dictionaries
Arabic Ontology + 150 Arabic dictionaries - The largest Arabic lexical database
The American Heritage Roget's Thesaurus. (2014). Boston, MA: Houghton Mifflin Harcourt Publishing Company - A well-known book of synonyms.
Martin, E. A., & McFerran, T. A. (Eds.). (2008). A dictionary of nursing. Oxford University Press. - over 10,200 entries).