Take a look at how Debra Dalgleish describes it at:
http://www.contextures.com/xlDataEntry02.html#Fill

leo wrote:

I have an xllss with 1000 rows and in column "A" is the customer name. In
many cases the customer name has blanks below it before a new customer name
appears. I need a formula that will automatically fill in the customer name
above but be smart enought to recognize a new customer name and then fill in
that name in the blank cells below it. in the example i need to auto fill abc
in row 2 and def in row 4.
customer
1 abc
2
3 def
4

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Dave Peterson