Math terms that start with the letter "J" include "Jacobian," "Jordan curve," "Jordan canonical form," and "Julia set." All of these terms come from higher math and are named after someone whose last name started with "J."
There are many math terms that start with "J," and most of them are named after famous mathematicians. One exception is "jump discontinuity" which occurs when someone drawing the graph of a function on a piece of paper must pick up the pencil to accommodate a sudden change in the function.
Another example is "Jacobian," which is introduced in multivariable calculus and refers to a calculation which helps determine the shape of a graph of a function of two variables. A Jordan curve is a simple, closed curve drawn in the plane. A circle is an example of a Jordan curve. Jordan curves divide the plane into two regions, one inside the curve and one outside the curve.
The Jordan curve is not to be confused with the "Jordan canonical form," which is a particular way of writing a matrix in linear algebra. Another mathematical term is "Julia set," which describes the set of points in the complex plane which do not diverge to infinity under iteration of a particular complex polynomial. Julia sets are studied in fractal geometry, and their colorful and complicated images are well-known even to many non-mathematicians.