Defined Names is an Excel feature where a name is assigned to a cell or range cells. How to name cells is at the Defined Names page

Naming rules:

• Names must begin with a non-number; a letter of the alphabet, the underscore ( _ ) character or the backslash ( \ ) character
  
• The names may contain numbers, but must begin with text
• Periods ( . ) are allowed
• Upper and lower case letters are interpreted the same in Excel.
  SalesReport, salesreport, and SALESREPORT are all the same name. By creating a name using caps, such as SaleReport, you may find it a bit easier to work with.
• Spaces and most punctuation character are not allowed.
• The underscore character helps to display a name with the visual spacing provided by the underscore character. Examles:
  Sales_Report, UnitsSold_Jan, March_Payroll, _Commissions
• A name can contain up to 255 characters.
• Many times Excel will prompt you when trying to use a non-allowable name
• The name cannot look like a cell reference
  For example, you desire to name your payroll summary W2 (IRS form W2).
  Excel will think this is cell W2.
  
• C, c, R, r cannot be used as names. Excel reserves for selection shortcuts