In mathematics we define estimation is a guess or finding the closest answer to the actual value through though or rough calculation. Rough calculation includes rounding off, using round figures such 100s and 1000s.
In math, the antonym of estimation is calculation. In mathematical terms this would be defined as determining the answer of a problem using mathematical methods including addition, subtraction, multiplication, division and other mathematical equations used to find the actual answer.
Let's illustrate this through an example. In an exam if we were to count the amount of words in the essay we would generally count every single word in each line, write them at the end of the margin in pencil and sum it up at the end of the essay. This is called calculation.
However, your examiner has 120 exam essays to mark and will not be able to count each line and tally the amount of words. The examiner will count the amount of words in the first line of the essay and multiply by the amount of lines in the essay. This is called estimation.
From the above example the estimation is an approximate answer to the true value, but calculation can give you the exact answer. (always read the context of the question or the use of the word in order to understand what is required)