Use of the term "outcome" I know that a particular value a variable can have is called an outcome.
Sometimes I see that a dependent variable is also called an outcome. Is the latter use a legitimate practice or bad use of the terminology?
 A: The terms outcome and response are common alternative terms for dependent variables. Far from being bad or illegitimate in any sense, they are excellent, evocative terms, underlining the origin or role of the variable, as the outcome or response (result, consequence) which we typically wish to describe, explain and/or predict. 
There is negligible risk of confusion between this use and the use (e.g. in probability) to refer to possible outcomes, say that a coin toss may yield outcomes of heads or tails. Indeed, these uses are consistent: outcome variable and individual outcomes both refer to observed results in some system. 
The terms that are poor and deserve to be avoided are dependent and independent variables. As for example Frederick Mosteller and John W. Tukey pointed out in their Data analysis and regression (Reading, MA: Addison-Wesley, 1977) these terms are already heavily overloaded, given uses for deterministic functional dependence in mathematics and their use in probability and statistics to characterise both dependence or independence of variables and dependence or independence of individual values. It's also not trivial that the words are so close that many learners get confused on which is which, causing temporary but disconcerting communication problems, as often seen in threads in this forum. Furthermore, as independent variables are rarely independent of each other, as needs to be explained as soon as (for example) regression is taught properly, it seems strange to have to introduce a term and then immediately to have to explain what it does not mean. 
A: Statistics is a "helper discipline" and as a consequence it is divided across different "substantive" disciplines (think econometrics, psychometrics, etc.). All of these have developed over time their own terminology. In my experience the most constructive way of dealing with this is to just be aware that the same word can mean different things and the same thing can have different names, mainly, but not exclusively, depending on which (sub-(sub-(sub-)))discipline the author is from.
