An approximation is a rough estimate or guess at something. When you write a budget for the month, you're making an approximation of how much you're going to spend. You can't be certain of the exact amount.
Like approximately or proximity, approximation comes from the Latin proximus which means "nearest." An approximation is the nearest estimate you can get without having the precise size or measure of something. It's a ballpark figure. When you set out for work in the morning, the amount of time you give yourself to get there is an approximation. Depending on traffic and weather, you can never know exactly how long the trip will take.