Android phones contain an extensive dictionary, but there will still be occasions when a word in a user’s lexicon won’t be recognized. These may be proper names (or improper names), trade terms, or words in languages other than the default language. There are several ways to add words to this dictionary; some more convenient than others. However, as it is with all things Android, the method for each device, model, and version may differ (at least slightly) from others.
In Android 4.0, one quick way is to type the word, and then tap the displayed word in the suggestions that appear above the keyboard, and tap it again to save it to the dictionary.