isogram

A word in which no letter of the alphabet occurs more than once.

For convenience, we can refer to an isogram of order n as an n-isogram. The examples in (2a) are 1-isograms, (2b) 2-isograms, and (2c) 3-isograms. Second- and third-order isograms are also sometimes referred to as pair and trio isograms respectively, but I will not use these terms here.

A line on a map or chart, such as a contour line, joining points that have the same value for some quantity.