Is this Word 2003 or earlier? If so, type the word, format it (if desired),
select it, then choose Insert - AutoText - New. You can also create entries
directly, using Tools - AutoCorrect Options - AutoText tab.
Another "flavor" of this are AutoCorrect entries (see AutoCorrect tab
instead of AutoText tab). AutoCorrect entries automatically expand a
shortcut into some larger form (e.g. usa might automatically expand into
United States of America). The difference is that with AutoText
(AutoComplete), you have to perform some kind of action to expand the item,
while AutoCorrect items automatically expand upon typing a word separator of
some kind (space, period, comma, semi-colon, colon, paragraph mark,
parentheses, and so on).
If you're using Word 2007, AutoComplete is limited to dates. AutoText still
can be expanded using F3. AutoCorrect still works in Word 2007 as it does in
previous versions.