In computer programming, a variable is a place holder whose value can change from time to time. For example, in C/C++:
int a; /* a is a variable of integer type */
a = 10; /* the value 10 is assigned to the variable a */
a++; / * the variable a is increased by 1; now a is 11 */
In mathematics, a variable is a value that you want to supply as an input to a function. For example,
f(x) = x + 10
Here, x is a varibale. It is sometimes called a parameter.
See Variable - Wikipedia, the free encyclopedia for more information.
In a research environment, a variable is a setting that can affect the outcome of your experiment. For example, let us consider the following chemical equations.
A + B = C (at temperature 25 degree Celsius)
A + B = D (at temperature 100 degree Celsius)
Here, "temperature" is an important variable/parameter in your experimental setup.