A nightmare is the sort of
dream that can be so
enveloping that you don't
realize it isn't
real. It is the perception of some
fear through a realistic
medium or
media, usually visual. Within the nightmare, you will characteristically be
chased or
stalked by some
abhorrent device (
man,
machine,
clown, etc.) from which you cannot
escape.
Some people will realize that they are within a dream and
attempt to escape the dream rather than the dream's
antagonist. This attempt may be
effective, because once efforts are directed toward
waking up, the
dream sequence has virtually no
consequence on your perception. At that point, it is possible to
whisper quietly so as to wake another in the room with you, or to merely contract your muscles, jolting you
awake.