If you choose Edit > Undo the last editor action performed is undone. Successive use of Edit > Undo revokes more actions (rather than undoing the last Undo command, as is the case with many other editors).
When using Emacs emulation you can undo via the Emacs keystroke
Ctrl+_
. Thus, to undo the last five words typed, press
Ctrl+_
five times.
On Mac OS X you can undo via the keystroke
Command+Z
.