The US National Cancer Institute (NCI) offers three English dictionaries:

• Dictionary of Cancer Terms (9,333 terms related to cancer and medicine).
  All entries include definitions and pronunciation in audio format.
• Dictionary of Genetics Terms (242 terms related to genetics).
  All entries include definitions and pronunciation in audio format.
• Drug Dictionary.
  All entries include brief descriptions of the agent/drug product and links to heck for active clinical trials using this agent and view this agent in the NCI Thesaurus.

(You can use the links above to access the dictionaries separately or click "Open resource" to open the page on NCI website with links to all dictionaries.)

To switch between English and Spanish, click the language button in the upper right corner of the web page. See the screenshot below: