xyaku is a general dictionary/search engine front-end for X.

xyaku invokes an add-in command by hitting a specified key
combination, and sends the contents of X selection buffer into
add-in's stdin, and receives the output from its stdout, then shows
the result in a pop-up window.

WWW: http://www.sepia.dti.ne.jp/inoue/software/xyaku/