The only "built-in dictionary" is one the iPad uses for spell checking and shortcuts. See the following from the iPad Manual:
Auto-correction and spell checking
For many languages, iPad uses the active dictionary to correct misspellings or make suggestions as you type. When iPad suggests a word, you can accept the suggestion without interrupting your typing. For a list of supported languages, see http://www.apple.com/ipad/specs
Accept the suggestion: Type a space, punctuation mark, or return character.
Reject a suggestion: Tap the “x” next to the suggestion.
Each time you reject a suggestion for the same word, iPad becomes more likely to accept the word.
iPad may also underline words you’ve already typed that might be misspelled.
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Shortcuts and your personal dictionary
Shortcuts lets you type just a few characters instead of a longer word or phrase. The expanded text appears whenever you type the shortcut. For example, the shortcut “omw” expands to “On my way!”
Create a shortcut: Go to Settings > General > Keyboard, then tap Add New Shortcut. Prevent iPad from trying to correct a word or phrase: Create a shortcut, but leave the Shortcut
field blank. Edit a shortcut: Go to Settings > General > Keyboard, then tap the shortcut.
Use iCloud to keep your personal dictionary up to date on your other iOS devices: Go to Settings > iCloud and turn on “Documents & Data.”
Aside from that, you can probably find a (free) dictionary in the iBooks Store.