We've found 3 phrases for approximation (0.116 seconds):

ballpark estimate »    A ballpark figure, a very rough approximation.

hand waving »    Discussion or argumentation involving approximation, vagueness, educated guessing, or the attempt to explain or excuse vagaries.

round off »    To change a number into an approximation having fewer significant digits.

New:  We also know Zip Codes FYI!