Answer:
In Math, a variable is a letter or symbol that represents some unknown value.
It's usually represented with the letters x or y.
For example, in x + 1 = 3, the x is the variable.
We can easily resolve that equation and know that the x has to be 2.
But is called "variable" because it can take infinite values when it's in a function.
For example, y = x + 1
y = 2, if x = 1
y = 3, if x = 2
And so on.
Therefore, the y depends on the x. That's why is called variable.