OverTilde[expr] displays with a tilde over expr.
OverVector[expr] displays with a right vector over expr.
PageBreakAbove is an option for Cell which specifies whether a page break should be made immediately above the cell if the notebook that contains the cell is printed.
PageBreakBelow is an option for Cell which specifies whether a page break should be made immediately below the cell if the notebook that contains the cell is printed.
PageBreakWithin is an option for Cell which specifies whether a page break should be allowed within the cell if the notebook that contains the cell is printed.
PageFooters is an option for notebooks that specifies what should be inserted as the footer of each page of a notebook when it is printed.
PageHeaders is an option for notebooks that specifies what should be inserted as the header of each page of a notebook when it is printed.
ParentDirectory[] gives the parent of the current working directory. ParentDirectory["dir"] gives the parent of the directory dir.
PasteBoxFormInlineCells is an option for cells that specifies whether a new inline cell is created when a typeset expression is pasted into a non-box-type cell.
Path
(Built-in Mathematica Symbol) Path is an option for Get and related functions which gives a list of directories to search in attempting to find an external file.