If you don't know where to start, these can be a good place to begin.
Two of the best such works are:
Audi, Robert, ed.. The Cambridge dictionary of philosophy.
2nd ed. Cambridge: Cambridge University Press, 1999.
Ref. B 41 .C35 1999
Honderich, Ted, ed. The Oxford companion to philosophy.
2nd ed. London: Oxford University Press, 2005.
B 51 .O94 1995
Other one-volume general reference works in our collection include:
Bales, Eugene F. A Ready Reference to Philosophy East
and West. Lanham, MD: University Press of America, 1987.
B 72 .B34 1987
Brugger, Walter and Kenneth Baker. Philosophical Dictionary.
Spokane, WA: Gonzaga University Press, 1972.
B 43 .B713
Burr, John R., ed. Handbook of World Philosophy: Contemporary
Developments since 1945. Westport, CT: Greenwood Press, 1980.
B 804 .A1 H36
Grooten, J., and G. Jo Steenbergen. New Encyclopedia of
Philosophy. New York: Philosophical Library, 1972.
B 48 .D8 N4313 1972
Macrone, Michael. Eureka! What Archimedes really meant and
80 other key ideas explained. New York: HarperCollins, 1994.
B 68 .M29 1994
Mautner, Thomas. A Dictionary of Philosophy.
Oxford: Basil Blackwell, 1996.
Ref. B 41 .M38 1996
Reese, William L. Dictionary of Philosophy and Religion:
Eastern and Western Thought. New and enlarged edition. Atlantic
Highlands, N.J.: Humanities Press, 1996.
Ref. B 41 .R43 1996