To update Webster 1913's definition of an altitude for a geometric figure (2D or 3D):
 A perpendicular line segment from a vertex to the plane extension of the opposite face.
 A perpendicular line segment from a vertex to the line extension of the opposite side.
 A perpendicular line segment between parallel faces or sides, or between a face and a parallel side.
 The line extension of such line segment.
 The length of such line segment.
A pictoral example of an altitude (def. D) of a
triangle ABC.


C
/\
/ a \
/ l \
/ t \
/ i \
/ t \
/ u \
/ d \
/ e \
/  \
A+B


Sources: (1) MerriamWebster's Collegiate Dictionary © 2001, (2)
"Vectors and Transformations in Plane Geometry" by Philippe Tondeur, Publish or Perish, Inc. 1993