The Purge command lets you free memory used
by the Undo command, the History palette, or the clipboard.
Choose Edit > Purge, and choose
the item type or buffer you want to clear. If it is already empty,
the item type or buffer is dimmed.
Note:
The Purge command permanently clears from
memory the operation stored by the command or buffer; Purge cannot
be undone. For example, choosing Edit > Purge >
Histories deletes all history states from the History palette. Use
the Purge command when the amount of information in memory is so
large that Photoshop performance is noticeably affected.