This page describes some Japanese-English dictionary software for Mac. (If you have not enabled Japanese input on your computer, visit the first page of this site first to find out how.)
Recent versions of Mac OS comes with a dictionary application (called simply "Dictionary."). Among the dictionary files it includes are several Japanese options: a large Japanese dictionary, a usage dictionary that distinguishes similar Japanese words, and an English-Japanese/Japanese-English dictionary. (These are Daijisen, Ruigo reikai jiten, and Progressive waei eiwa chûjiten, all published by Shôgakukan.) If you can read enough Japanese to use all three of these (the first two are Japanese only), then this is a formidable combination.
The Japanese dictionaries are not activated by default; to enable them, you must open the Dictionary application, then open its preferences from the file menu and check the dictionaries you want to use. Drag the Japanese-English and Japanese dictionaries so that the one you prefer to use as a default is on top.
Other Japanese Dictionary Software
There is also a wide range of third-party dictionaries with features keyed toward Japanese language learners, like integrated kanji dictionaries and the ability to create vocabulary lists. Good ones include the elegant Jisho, the feature-rich JEDict, and the open-source Tagaini. Most of these programs use the same publicly available dictionary files (the learner-oriented EDICT/JMDICT dictionaries developed by Jim Breen at Monash University), so the definitions will be the same, but the interface and features vary. Some of these programs also allow you to load additional dictionaries downloaded from sites like the Monash ftp archive. A really useful one is ENAMEDICT, a free public dictionary of Japanese names.
Using the Dictionaries with Browsers, Email, etc.
Try this: highlight a word in Safari and select "Look Up in Dictionary" from the Services menu (under the Safari menu just to the right of the apple menu): Apple's Dictionary application opens and displays a definition of the word. Or control click on the word in Safari and select "Look up in Dictionary" from the pop-up menu, or just press the Dictionary shortcut command-control-D: A pop-up window will open with definitions. You can also use this trick in other applications, though some applications support services better than others.
Third-party dictionaries can also invoked like this from the Services menu. After you install a program like Jisho, for example, will find an item "Look up in Jisho" appears the services menu, with its own keyboard shortcut.
If this option does not appear in the services menu, or if the shortcuts don't work, you may need to tweak some things in the System Preferences. In recent versions of Mac OS, go the Apple menu and select "System Preferences," then select the "Keyboard" preferences pane, click on the "Keyboard Shortcuts" tab, select "Services" in the lefthand pane. Find the relevant shortcut in the righthand pane, and make sure it is checked. If the keyboard shortcut is not working, you can double click directly on its keyboard shortcut and assign another key combination.
Dictionaries on the Web
Many English-Japanese dictionary web sites oriented toward learners use some varient of Jim Breen's EDICT/JMDICT dictionary described above. Breen's own WWWJDIC web site is rather complex, but provides access to all of that dictionary's functionality.
Two other dictionaries available on the web are goo and Eijirô:
NTT's goo dictionary incorporates several large dictionaries, including the Daijirin Japanese dictionary and the EXCEED J/E and E/J dictionaries (all published by Sanseidô). There's a Firefox plug-in that integrates with the goo dictionary.
Eijirô, accessible from ALC's Home Page, is a large proprietary dictionary assembled by translators and users, said to be particularly strong on technical terms. (It has more than half a dozen entries beginning with "deconstruct," for example, including one for the Woody Allen movie Deconstructing Harry.) To look up a term in Japanese, you need to enter it in kanji.