Sometimes we call it variable or unknown.
In x+2=9, x is variable but we can work out if we try!
A variable does not have to be ‘x’ it may be ‘y’ ‘w’ ‘z’ or any letter named or symbol.
The above question could be asked what does x mean in math.
Examples:1 what is x
Find the value of x, if x+9=4.
subtracting ‘9’ on both sides