Theory:

Why we need estimation?
 
Suppose your school arranged an annual day celebration. The first thing they think about is finding the rough number of people expected for this celebration.
 
Here the question is, Is it possible to get the exact number of visitors to the celebration?
The answer is, 'Not possible'.
 
So we need a smarter tool to get them close enough number.
In day-to-day life, we heard the following types of lines.
  • Nearly \(50,000\) people watched the India-Pakistan cricket match at M.A. Chidambaram Stadium, Chennai.
  • The Black Death, also known as the Plague, was the fatal pandemic recorded in human history, resulting in the deaths of about \(200\) million people in Eurasia and North Africa.
  • On December \(28\), \(2011\), the U.S. Navy’s Joint Typhoon Warning Center (JTWC) reported that Thane Cyclone was located roughly \(270\) nautical miles (\(500\) kilometres) southeast of Chennai.
All the above news items don't give exact numbers. 
 
The number mentioned is not accurate. They are closer to the exact number.
 
The actual value of the first example might \(49650\), \(50120\), \(49980\), \(49702\) or \(50348\). 
 
Similar way the actual value of the second case could be \(200.6\) million,  \(199.7\) million, \(199.2\) million or \(200.4\) million people died, and the third value could be \(268\), \(274\), \(266\), \(271\) or \(273\). 
 
Note that they all are nearest to the numbers. Not the exact numbers. 
 
To find these situations exactly, you have some keywords like 'nearby', 'approximately', 'about', 'roughly' or 'about'. 
 
Thus, it can be concluded as follow:
Estimation is finding a number that is close or nearby to the exact number.
Important!
To get the estimated value, we round off the number to nearest tens, hundreds, thousands, lakhs/millions, and so on.