While words related to math that start with the letter J are uncommon, certain two-word phrases starting with J can be found in the modern mathematician's lexicon. Among these are "jump discontinuity" and "joint variable."
A jump discontinuity occurs when the data used to formulate a graph define separate populations on the plotted graph. This results in a split between otherwise contiguous plotlines. Jump discontinuities are strictly defined as cases in which the limits from the left and right both exist on the graph, but are not equal to each other, according to Mathwords.com. When plotted against a graph, data sets showing a jump discontinuity appear as discrete data sets that are not in contact with each other, as with a line that does not intersect a curve.
Joint variations are sometimes known as jointly proportional variables. The term "joint variations" describes a set in which a variable, z, is directly proportional to all of the variables represented on the opposite side of the equation. An example of this condition, given by Mathwords.com, is c = 5ab. Plotted as a data set, variable c is directly proportional to each of the variables a and b. This makes c jointly proportional to both a and b together.