parameter  n. 

1. An independent variable appearing in a mathematical expression, which may be assigned values (calculated or assumed), each of which is held constant to determine the corresponding specific form of the expression. 
2. Informal: (a) A fixed limit or boundary. (b) A characteristic or defining element. 

P = T / N

Both T and N are called "variables."  They are independent of each other.  For a given book, it is the author or the editor who determines T.  The book designer decides on N.  Knowing T is not enough to calculate P.  If N is not known, you're stuck.

There are a couple of ways to assign a value to an independent variable to make it into a "parameter": by calculation and by assumption.  Take N, for example, the number of words on a page.  Its value varies with such things as page size and type face.  If they are not known, an arbitrary value for N might have to be assumed, just to get on with it.  Either calculated or assumed, N would then be fixed, and the number of pages P varies only with T, the length of the text.  N is said to be a "parameter."  Although it is a "variable," N is holding still while T varies, you see.

Generally, the length of the text T will already be known.  If so, the same mathematical expression, P = T / N, gives the number of pages P for various values of N, the number of words which fit on each page. T has become a "parameter" -- a "variable" which is held constant.

 "Parametric Reasoning," the elementary thought process illustrated above, affords great advantages to those who must deal with quantitative problems of whatever complexity.  The mathematical meaning of "parameter" (sense 1) closely aligns with ceteris paribus ("other things being equal") -- a powerful discipline that has been proven in many realms.  Instinctively, we admonish ourselves to hold everything constant and to vary only one thing at a time.  Thus do we explore problems with our minds, ever alert for "concomitant variations," from which we nominate causes for an observed effect.

Then too, "arbitrary" values for parameters most often result from an ordered set of assumptions, also called a "model" -- arguably the most beneficial application of computers.  Accordingly, Parametric Reasoning provides a numerical means by which to make assumptions explicit -- the most powerful intellectual instrument of discovery yet discovered!

The more recent, non-mathematical meanings of parameter (senses 2a and 2b) are widely disputed.  Examples abound:

• The publisher was forced to stay within the parameters (fixed limits) of the dictionary's budget.
• The author attempted to widen the parameters (boundaries, in a more general sense) of the dictionary's content.
• Diversity was a parameter (characteristic or defining element) of the dictionary's entries.

Parameter is a distinguished word with a precise meaning, which has been appropriated and exploited by pretenders.  The term has declined into the means only for self embarrassment...

"Let's go over the parameters of the problem -- whatever that's supposed to mean, ha ha."