The axis of ordinates is the vertical line, usually represented by the letter Y, which is part of a Cartesian coordinate system.
From the first subjects taught in the school on mathematics, the Cartesian system is used to represent points or variables on a plane. Its name comes from the philosopher, who also mathematician, René Descartes.
The system is used to represent, above all, mathematical functions or the relationship between two variables.
The ordinate axis is indicated in green. That is, it is the horizontal line of the plane. The letter Y is attributed to said axis by agreement. So, mathematically, it would not be wrong to assign any other letter. For example, we could say that the variable U is represented in the ordinate axis. Therefore, it would make sense to call it the U axis or the U axis.
Of course, to avoid confusion, the letter Y is used. In any case, it is not convenient to get used to using that nomenclature, since in most cases other variables are used. In the absence of variables, it is always better to call it an ordinate axis.
The quadrants and the ordinate axis
In order to better assimilate the concept, we will see what sign the ordinates have according to the quadrant in which a point is found.
Depending on which quadrant a point is located, the ordinate axis will have a positive or negative sign. We must learn to interpret and visualize, not learn the signs by heart. In the case of the Y axis (ordinate), any point above the abscissa axis (X axis) has a positive sign. On the contrary, every point under the X axis has a negative sign.