Definition:Vertical

Vertical Line
Hence the adjective vertically, meaning in a vertical line.

Also presented as
The term the vertical is frequently seen, particularly in the context of specifying the spatial orientation of a given (straight) line: the angle it makes to the vertical.

In the context of the presentation of mathematical graphics on, for example, a piece of paper arranged horizontally, the vertical is that direction towards and away from the reader. Thus such a direction would be actually vertical if that paper were held in a vertical plane oriented for normal reading.

It is of course noted that it is difficult at this level to define vertical and horizontal without the definitions at some point becoming circular. An appeal to an intuitive understanding is therefore necessary to a certain extent.

Also see

 * Definition:Horizontal