CellBaseline is an option for Cell which specifies where the baseline of the cell should be assumed to be when it appears inside another cell.
CellBaseline is used to determine the vertical alignment of cells that are embedded in text, typically in TextData objects.
CellBaseline->pos specifies that position pos in the Cell should be assumed to be the baseline of the Cell and should therefore be aligned with baselines of other boxes.
Possible settings are:
See The Mathematica Book: Section 2.10.9.
See also: CellMargins, GridBaseline.