Slope
From AoPSWiki
The slope of a line can be intuitively defined as how steep the line is, relative to some coordinate system. To be more precise, given a line 
in the Cartesian plane and two points, 
and 
, on with 
, the slope 
of is equal to 
If all points on have the same 
-coordinate (abscissa), we say that has infinite slope.
Other expressions for the slope are
|
 (read "delta  over delta "),
|
or  .
|
If 
is the directed angle between the -axis and , the slope is also given by 
.
