A symbol, usually a letter, that is used to represent 1 or more numbers
A variablẹ
it is called a variable not a letter
It is a variable
It is a variable
X and y
The symbol is ρ (the lower case Greek letter rho). Or the Latin letter D can also be used.
The sign or abbreviation to represent an element is called an "atomic symbol", and there is a distinct one for each element. Every atomic symbol begins with a capital letter, which may constitute the entire symbol but is usually followed by a lower case letter.
variable
He used "aleph", the first letter of the Hebrew alphabet, which looks like this: א
If you are talking about math then its usually " x " but if that is already use in the expression and you want to use another symbol to represent a DIFFERENT number then you can use y or a or so on.
the letter J