2-Dimensional space (commonly abbreviated as 2D) is any space with 2 axis points of reference. The most common axis are X (left-and-right) and Y (up-and-down). Any other direction in 2D space can be seen as a combinational movement along these two axes (i.e. a horizontal movement/line is really just +/-X and +/-Y).
A line within 2D space has a slope (M), defined by
Y2 - Y1 M = _________ X2 - X1
- wikipedia: 2-D
- wikipedia: Traditional animation
- Slope Formula: http://math.about.com/library/blslope.htm