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1. To increase, by some additional amount, a quantity that is associated with something: The waiter added $5 on the bill for the extra pretzels we ordered. That tip is too low for the good service we had, so let's add on another dollar. Last night, the storm added on more than twenty inches to the record snowfall.
2. To attach something in order to extend or enlarge something else: We're adding another room on the back of the house. The guest list is full, so we can't add anybody else on. If there aren't enough logs in the fire, add another one on.