[Math] mathematical symbol for unknown

notation

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.

Best Answer

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

and

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

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