# Is there a mathematical symbol for unknown?

I'm searching for two symbols - considering they exist - (1) unknown value; (2) unknown probability.

Note: I thought that $x$ was used in a temporary context, whenever I see it, it remains unknown until an evaluation is made. I was thinking in a "unknown and impossible to be known" context. I'm not sure if this context exists or if $x$ also express it.

-
$x{}{}{}{}{}{}{}$ –  t.b. Sep 12 '12 at 12:26
You can choose any symbol you like actually. –  ᴊ ᴀ s ᴏ ɴ Sep 12 '12 at 12:58
With regard to the edit: It is impossible to know whether $x=1$ or $x=-1$ in $x^2=1$. –  Michael Greinecker Sep 12 '12 at 13:01
We must know what we are talking about. That is by definition we must be able to express it. Else, we do not talk about the quantities we don't know. However, some things can be expressed in a limited way, and yet cannot be evaluated. For example, we do not talk about infinite sums, we only talk about limits of infinite sums. So, what is the sum of an infinte series? We say we can not evaluate it, however, if you take so and so many finite terms, the series will be withing this range of the "limit". In that case, then we can use $x = \lim_{n \rightarrow \infty} \sum a_{n}$ –  Jayesh Badwaik Sep 12 '12 at 14:17
@MichaelGreinecker It's impossible, but since $x^2=1$ for $x=1$ and $x=-1$, can't we assume that we have a set of two numbers that answers the question $x^2=?$ and thus accepting it as possible to be known? –  Pristine Kavalostka Sep 13 '12 at 0:49
show 1 more comment

As the others already mentioned. There are conventional variables to the unknowns such as $x$ for an unknown value or $P(A)$ for an unknown probability. We take them as variables and assume that they are unknown however there is a certain probability that they take some reasonable values. That is actually the reason why we define them.

Additionally in computer programming there is a term called 'NaN:=Not a Number'. That is also somehow an unknown value, However even worse, we dont have any hope that it can be as in the case $x$. They are often mentioned as Indeterminate_forms arising from $0/0$, $\infty-\infty,\infty/\infty$ etc..

There might be also another interpretation to understand an unknown probability. If you have a probabilistic model and If you have deviations from the model assumptions due to outliers or inaccurate estimation etc.., then this phenomenon is called uncertainty. In such a case you are unsure about which probabilistic model you have resulting an unknown probability.

-

Generally used Symbol for unknown value is $x$ and other letters.

and

unknown probability of some event $A$ is denoted as $P(A)$ or $P_A$.

-
One convention of notation is to use symbols from near the end of the alphabet (e.g., $x$, $y$, $z$, $t$ as others have answered) for "unknown" values and symbols from near the beginning of the alphabet (e.g., $a$, $b$, $c$) for "known" or "fixed" values. Of course conventions vary between fields of mathematics.