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If you are referring to bisection search eg. in Problem Set One 'epsilon' is a variable that is the difference between the guessed answer and the actual answer.
As the program iterates towards a solution epsilon gets smaller
It's not in python--it's in Calculus.
In computing (as opposed to Calculus) epsilon is some small positive value such that if you are within that range of the actual solution, it is close enough and acceptable for your use. You can't get the actual value of an irrational like square root of 2, but you can get close and if you choose your epsilon to be small enough, you can use the calculated value in place of the square root of 2.
Now, you can't choose epsilon to be arbitrarily small (in the infinitesimal range). It might be so small that the computer can't get close enough to the desired solution and instead will bounce around trying to get close but will keep overshooting the solution.