Here are some free online dictionaries. Given the dynamic nature of computer science, online dictionaries are preferable to print:
FOLDOC: Free On-line Dictionary of Computing http://foldoc.org/ " includes acronyms, jargon, programming languages, tools, architecture, operating systems, networking, theory, conventions, standards, mathematics, telecoms, electronics, institutions, companies, projects, products, history etc. in fact anything you might expect to find in a computer dictionary. " (source: Home page).
TechWeb's TechEncyclopedia from TechWeb
Webopedia™ http://www.webopedia.com A dictionary for words, phrases and abbreviations related to computer and the Internet with the intenetion of avoiding jargon and for all levels of computer experience and every definition is verified among multiple sources. New terms are added on a daily basis.
Mathematical Programming Glossary http://glossary.computing.society.informs.org
Netlingo http://www.netlingo.com/dictionary/all.php Online Dictionary about the Internet.