It would be called a variable, and can be symbollized by anything, but the letters x and y are common.
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There is no specific symbol. The symbol for real numbers is R and that for rational numbers is Q so you could use R \ Q.
The symbols used for counting in math are called numbers, or numerals. They can be whole numbers, or fractions of parts of a number.
That is an ordered number pair. It is normally used to specify a location on a graph, the numbers indicating the values of x and y, in that order.
The method used is called scientific notation.
That symbol is called the radical.