Can I only consider my book a bestseller in it’s category, or can I use search terms too?

There’s no official rules for what constitutes a best seller, so you can pretty much do what you want. For example, the New York Times has a few bestseller lists based on sales of hardcover books, which is the criteria that they use.

Most Amazon publishers seem to consider the bestseller lists for overall and category because those are the lists that Amazon creates, and that’s what I tend to look for. I’ve never looked at specific search terms and how books are selling for that term, and wouldn’t personally consider myself a bestseller because I rank well for a specific term, at least not without qualifying the statement and explaining it which is more trouble than it’s worth.

If your book has sales and reaches any of the overall or bestseller rankings then you should be safe considering yourself a best seller. I’d recommend that you take a screenshot that proves it; then you have something to show off in a blog article if you ever want to publicize the fact, and if somebody calls you on it you have the historical proof that you were a bestseller (since obviously a book may sell better at one time than at another.)