The straight line which cuts the three scales of a nomograph at values satisfying some equation. Also known as index line.

1. A line of equal or constant value of a given quantity with respect to either space or time. Also known as isogram.

2. More specifically, a line drawn through points on a graph at which a given quantity has the same numerical value (or occurs with the same frequency) as a function of the two coordinate variables. Also known as isarithm.