Johnson solid
A convex solid whose faces are all regular polygons but that is not a Platonic or Archimedean solid. A square pyramid with equilateral triangle sides is one example.
Joint frequency
In a two-way table, the count in a single cell: how many data values have one category of each variable at the same time, such as boys who answered yes.
Joint probability
The probability that two events happen together, written P(A and B). For independent events it equals P(A) × P(B).
Joint variation
A relationship in which one quantity varies directly with the product of two or more others: y = kxz. The area of a rectangle varies jointly with its length and width.
Jordan curve
A closed curve that does not cross itself. The Jordan curve theorem says such a curve always splits the plane into an inside and an outside.
Joule
The SI unit of energy or work: the work done when a force of 1 newton moves an object 1 meter. One food Calorie is about 4,184 joules.
Jump discontinuity
A break in a graph where a function jumps from one value to another, because its left-hand and right-hand limits are different.
Jump strategy
A mental math method that adds or subtracts by making jumps on a number line, often tens first and then ones: 47 + 25 is 47 + 20 = 67, then 67 + 5 = 72.
Juxtaposition
Writing two symbols side by side with no sign between them, which means multiply: 3x means 3 × x, and 2(5) means 2 × 5.