Oxford English Dictionary
The OED is the authoritative dictionary on the English language; the online version contains the 20 volumes of the print version, with revisions. Updates quarterly.Allows up to 15 simultaneous users.
Here are some free online dictionaries. Given the dynamic nature of computer science, online dictionaries are preferable to print.
General reference:
FOLDOC: Free On-line Dictionary of Computing "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)
Computer Language: self-published since 1978, this encyclopedia is the self-proclaimed most successful tech reference in history.
Webopedia™: a dictionary for words, phrases and abbreviations related to computers and the Internet, with the intention of avoiding jargon and for all levels of computer experience. New terms are added on a daily basis.
Specialized dictionaries:
NIST Glossary aggregatesterms and definitions specified in NIST's cybersecurity and privacy standards, guidelines, and other technical publications, and in CNSSI 4009.