Force Dictionary to Use One Window in Snow Leopard
One of the new "features" in Snow Leopard's Dictionary is that the built-in dictionary will open a new window for each word you define from either the Spotlight menu or the Services menu (Control-click on a word and select Look Up in Dictionary from the contextual menu). In OS X 10.5, these searches all opened in one window, and you could then use the arrow buttons in the toolbar to navigate back and forth between definitions.
If you'd prefer to have Dictionary in 10.6 open everything in one window, here's how to change its behavior. (I covered this tip in last week's Snow Leopard tips video, but I know not everyone watches the videos, so here it is in text form.)
Note that this hint is not related to looking up definitions in the floating dictionary window--the one that appears when you press Control-Command-D over a word in a Services-aware application. It's about the behavior of windows in the full Dictionary application.
First, quit Dictionary if it's running, then open Terminal. Copy and paste the following command, then press Return.
defaults write com.apple.Dictionary ProhibitNewWindowForRequest -bool TRUE
From now on, when you look up words in Dictionary, they'll all open in one window.
If you find that this isn't happening, even after running the command, it's probably because you had multiple windows open in Dictionary when you quit it the last time. If that's the case, just close all the open windows, and Dictionary should then start working in one window; see this brief movie for a demonstration of this issue and the fix.
If you ever want to go back to the multi-window method for Dictionary lookups, quit Dictionary and enter this command (and press Return):
defaults delete com.apple.Dictionary ProhibitNewWindowForRequest
That command deletes the preference forcing Dictionary to use one window; the next time you run it, it'll be back to its multi-window business-as-usual self.