TITLE: Are there 3 or 4 quartiles? 99 or 100 percentiles?
QUESTION [10 upvotes]: So I understand that a quartile is a quantile where the data is divided into four groups.
   1   2   3
---|---|---|---

And 1, 2, and 3 are the quartiles. The second quartile is the median, etc.
But while studying for the GRE, I read this as part of the answer solution to a question:

From this you can conclude that the word quartile refers to one of the four groups that is created by listing the data in increasing order and then dividing the data into four groups of equal size.

Which indicates that these are the quartiles:
 1   2   3   4
---|---|---|---

So which is it? Are there 3 quartiles or 4?
On the bounty: I have sufficient evidence that the term is overloaded, so any answer that glosses over that won't be accepted. I'd like an explanation of how to use the terms and when.

REPLY [3 votes]: The word quartile refers to both the four partitions (or quarters) of the data set, and to the three points that mark these divisions. After all, we can't have one without the other. 
When citing a value for a quartile, though, we are specifically referring to the three dividing points, else it'd be meaningless. Thus, the first, second, and third quartiles have a specific value in a data set. These points are often referred to as the lower, middle, and upper quartile.
On the other hand, we can say that there are multiple data points contained in the first, second, third, and fourth quartiles. In this context, we refer to the actual partition. 
It all depends on context. The word is malleable, but the intent ought to be clear when used properly.