I must agree with Mr. Varma on the definition for variable pay. And I must respectfully disagree with your teacher Kruthi. However I would like to add a dimension to variable pay that seems to be overlooked by many organizations.
In a layman's language, if a variable pay scheme exists, then a person will get 'extra' money depending on whether he has achieved, what he is supposed to achieve & whether the company has done well. Some companies will reward the employee even if company is not performing but if the employee is performing. So if for example 85% of an employee's package is fixed and 15% variable & if the employee's monthly salary is 100 Rs (say) then he gets 85 Rs irrespective of whether we worked or not (provided he was present in office) & the 15 Rs he will get if he achieved his targets.
Now the 15% incentive takes care of 2 things: a) That the targets for the company are achieved b) The employee is motivated to do earn more money by doing more work. So if hypothetically, the company has 5000 employees and assuming that all 5000 employees achieve targets (most of them do in real life) then the company pays out 5000 x 15 x 12 = 9 Lakhs per annum as incentive.
Now the dimension I'm adding is the company can actually save a lot of money here. And the answer lies in the second point I mentioned above which is employee motivation. As per research not every person is motivated by money. Each employee is motivated by different things. Some people are motivated by more responsibilities, some people are motivated by spending time in social service, some people by verbal congratulation by the CEO, some people by gits etc. Some famous models such as Holland's RIASEC model provide basis to this thought. So if a company identifies the type of employee and designs a variable pay to suit his/her interest, both the company & the employee wins!
Regards,
Mathew