A what is a letter or symbol that represents some unknown value

Unknown values can be taken as anything, for example : the most common is x for unknown values and sometimes, for angles we can take θ (theta).

it is totally upto you!

Variables are arbitrary unknown values that are used to represent a value that isn't constant. They are commonly referred to as 'placeholder' values. These are commonly also referred to as parameters when dealing with co-ordinate geometry.