The definition of a tarot deck varies quite a bit from person to person. The bare bones definition is a deck of 78 cards: 22 majors and 56 minors (of four suits, ace – 10, and four courts per suit).
There have been, historically, deviations. An example is a Minchiate deck, which has 40 majors, plus the Fool. Modern deck designers have played with this structure as well, sometimes adding a few cards to the majors or adding a court to the minors, or even an entire fifth suit. Some would argue these are not tarot decks.
I've never considered it much of an issue; a Tarot deck is a deck that follows the rules mentioned at the top of the article. I use the term "oracle decks" -- such as the Druid Animal deck, among others, -- to describe cards used in readings, but just not strictly a Tarot deck. I often prefer oracle decks; this fit my style, more fluid. But of course I used Tarot as well!