what is the definition of the term "incremental sales"?

Hello

"Incremental Sales are purchases made by customers in addition to what
they intended to buy when they entered your store. The tourist who
goes to a gift shop in a resort town looking for a trinket, but buys
an expensive bowl instead, is one example. The museum visitor, who
wants a book about the museum's exhibits, but also ends up buying a
gift for their child's teacher, is another example. The hospital staff
person, who intended to spend $25 on a gift for Aunt Sarah's birthday,
but spends $50 including two additional items, is a third example."

This extract is taken from a site that goes into great detail about
other things that Incremental Sales that you might find interesting.

Hope this Helps. 

Johnny Phoenix

Easy for the layperson to understand.