CellMargins is an option for Cell which specifies the absolute margins in printer's points to leave around a cell.
Possible settings are:
The left margin gives the distance from the edge of the window to the left-hand side of the cell.
The right margin gives the distance from the inside of the cell bracket to the right-hand side of the cell.
The left and right margins can be set interactively in the front end using the Show Ruler ruler.
The top and bottom margins determine the amount of space to leave above and below the cell.
The margins go to the edge of any cell frame that is present.
Cell dingbats are placed to the left of the left-hand side of the cell, and extend into the left cell margin.
See The Mathematica Book: Section 2.10.9.
See also: CellFrameMargins, CellBaseline, ImageMargins, WindowMargins.