An approximate value by defect of a number is a value that is close to this number, less than it, as close as possible, and with a requested level of precision.

• The number 3.1415 is an approximate value by defect of the number π.

An approximate value by excess of a number is a value that is close to this number, greater than it, as close as possible, and with a requested level of precision.

• The number 3.1416 is an approximate value by excess of the number π.