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Use Tree Data Structure

What kind of lookups will be required? A directory is usually implemented using something like a B tree, but depending on your application, you may also go for self-balancing trees/treaps etc. If you're simply looking for an alternative to a dictionary, then you can consider using a trie, or hashing.

Use hashing and store the whole dictionary in a binary tree...
every word will create a unique hash code and based on code store and retrieve the meaning (stored in leaf element).

Ex: hash code for a: 00000, b:00001... n:01101, ... z: 11010
Hash code for word 'an': 0000001101

In the binary tree traverse left if its 0 and 1 for right.

Create a Trie out of the dictionary. Search time will be O(c) where c is the number of letters in the word which you want to find regardless how many words is in the dict.