The Swiss
 artist Paul Klee
 defined line as "a dot out for a walk."
 Paul Klee,
Paul Klee, 
"They're Biting," 1920, 
drawing and oil 
on paper, 
31.1 x 23.5 cm.
Seriously, go get a dictionary and look up "line" (
n.) as it applies to drawing. The definition doesn't begin to describe mark-making and its possible dimensions. Webster's New World College Dictionary, Fourth Edition, provides 36 definitions for line as a noun, but only one comes close: "...a very thin, threadlike mark; specif., a long, thin mark made by a pencil, pen, chalk, etc." Klee's definition is much more accurate.
Still more about line tomorrow...