Rounding error
Error caused by representing and operating on numbers with finite precision.
Definition
Computers store real numbers in floating point with a fixed number of digits, so almost every datum and operation introduces a small error. These errors can accumulate or be amplified, especially when subtracting nearly equal numbers (cancellation).
How it is used
It is the flip side of truncation: as the step shrinks, truncation decreases but relative rounding grows, which imposes an optimal step in finite differences.