adjective or noun pre·ven·ta·tive \-ˈven-tə-tiv\
Definition of PREVENTATIVE
Now, what we have here is a perfect example of how the language changes over time. When you look up a word and the definition is another word, that usually means that the initial word is not the preferred term. As a matter of fact, it may not even be a word, but its usage has become so common and accepted that the dictionary has included it in the lexicon.
For example, several years ago you would not have found the word "irregardless" in the dictionary. It is there today, referring you to the correct term "regardless".
So it is with "preventative". It's not actually a word, but it has become so commonly (mis)used that it is now accepted.
However, be a friend of grammar and do not use it. Use the correct "preventive" instead.
Oh! Here is the definition and etymology of preventive:
a : designed or serving to prevent the occurrence of disease <preventive medical care>b : undertaken to forestall anticipated hostile action
First Known Use of PREVENTIVE