It can be the combined effect of two kinds of error in a calculation.
The first is referred to as Round-off error and is caused by the finite precision of computations involving floating-point numbers.
The second, usually called Truncation error, is the difference between the exact mathematical solution and the approximate solution obtained when simplifications are made to the mathematical equations to make them more amenable to calculation.
Floating-point numerical error is often measured in ULP (unit in the last place).
You can help Wikipedia by expanding it.This applied mathematics–related article is a stub.