Please remember that subject-specific reference sources provide background information on topics. They were written by experts in field and use the terminology or language of that discipline. They are a great place to begin for a quick introduction to topic. These resources may include:
There are many other reference materials that may be helpful and save you time. Please just ask for suggestions.
An online version of McGraw-Hill encyclopedia of science and technology. In addition to articles, this database includes a dictionary, research updates, biographies of leading scientists, a research guide for students, and links to Web sites.
Standard mathematical tables and formulae by 
Encyclopedia of Mathematics by 
The Facts on File Dictionary of Mathematics by 
The Encyclopedia of Integer Sequences by 
Handbook of Mathematical Formulas and Integrals by 
