A

absolute reference
cell reference which will not change if cells are moved. Has the format $A$1.
active
the cell which receives the action or keystrokes. It has a wider, dark border 
adjacent
parts that are next to each other
argument
a value needed to calculate a function. For example, the function COUNT needs cell references as the arguments: =COUNT(A2:D6) or =COUNT(B1, C2, D3, D4, F5)
assumed values
The values that are fixed in formulas on a sheet, like interest rate or commission rate.
AutoCalculate
A special feature of Excel which shows a calculation on the status bar. You may select from several, including SUM and COUNT.
AutoComplete
A feature of Excel which automatically completes certain words or phrases.
AutoFill
A feature of Excel which automatically copies a cell into cells that are dragged across, or which automatically continues a pattern of values in cells that are dragged across.
AutoFit
Clicking the edge of a column heading or a row heading makes the column or row as wide or tall as needed to show contents of the cell that needs the most space
AutoFormat
a set of formatting choices that can be applied all at once
AutoSum
a toolbar button which sets a formula to add up cells either in a row or in a column. Excel guesses which one you want.
Axes
edges of a chart which are marked off in even sections