"Oxford Student's Science Dictionary," published by Oxford Dictionaries, is an excellent science dictionary for ages 14 and up. It explains more than a thousand words in detail. Oxford also offers illustrated science dictionaries. Seymour Simon offers a more detailed solution as an online resource at seymoursimon.com/index.php/science_dictionary.
For a Google-like alternative, consider The Science Dictionary. It works in a similar manner to Google but only includes the websites that offer scientific definitions in its searches. It has over 13 trillion pages of scientific information indexed in its servers as of 2015 and is therefore a colossal resource. It can be accessed at thesciencedictionary.com.