A
line which is
perpendicular to the
horizon and passes through the
center of the nearest
planetary body. If you don't have a planetary body nearby, then
horizontal and
vertical are pretty
arbitrary and you can decide which is which for yourself.
In particular, this refers to the
pipe character ( | ), although it's really a
line segment. These are the
Freudian slips of
Everything since they let you write one thing but mean
another.