h
The abbreviation for height in formulas, such as A = ½bh. It can also stand for hour.
Half
One of two equal parts of a whole, written ½ or 0.5.
Half-life
The time it takes for a quantity that decays exponentially to fall to half of its starting amount.
Half-line
The part of a line on one side of a point, extending forever in one direction; similar to a ray.
Half-plane
The part of a plane on one side of a line. The line is its boundary.
Harmonic mean
For two numbers x and y, the harmonic mean is 2xy / (x + y). It is used for averaging rates, such as speeds.
Head
One of the two possible outcomes when you toss a coin. The other is tail.
Hectare
A metric unit of area equal to 10,000 square meters, about 2.47 acres.
Height
The perpendicular distance from the base of a figure to its top or opposite vertex; also called altitude.
Hemisphere
Half of a sphere, cut through its center.
Heptagon
A polygon with seven sides and seven angles.
Heron's formula
A formula for the area of a triangle from its three side lengths: A = √(s(s − a)(s − b)(s − c)), where s is half the perimeter.
Hexagon
A polygon with six sides and six angles.
Hexagonal number
A figurate number of the form n(2n − 1): 1, 6, 15, 28, …
Hexagonal prism
A prism with two hexagonal bases joined by six rectangular faces.
Hindu-Arabic number system
The number system used today, with the ten digits 0–9 and place value.
Histogram
A bar graph that shows how often data fall into equal intervals. The bars touch because the intervals are continuous.
HL congruence
Hypotenuse-Leg: two right triangles are congruent if their hypotenuses and one pair of legs are equal.
Hole (in a graph)
A single point missing from a graph where a function is undefined, shown as an open circle.
Horizontal asymptote
A horizontal line y = c that a graph approaches as x grows very large or very small.
Horizontal axis
The x-axis: the horizontal number line on a coordinate plane.
Horizontal line
A line that runs left to right, parallel to the x-axis. Its slope is 0 and its equation is y = c.
Horizontal line test
A test for whether a function is one-to-one: if any horizontal line crosses the graph more than once, it is not.
Horizontal shift
Sliding a graph left or right. The graph of y = f(x − h) is the graph of f moved h units right.
Hour
A unit of time equal to 60 minutes. There are 24 hours in a day.
Hundred
The number 100, which is 1 more than 99 and equal to 10 tens.
Hundred chart
A 10 × 10 grid of the numbers 1 to 100, used to find patterns in counting, skip-counting and place value.
Hundreds place
The third place to the left of the decimal point, showing how many hundreds a number has.
Hundredth
One of 100 equal parts of a whole, written 1/100 or 0.01. It is the second place to the right of the decimal point.
Hyperbola
A conic section with two mirror-image branches. It is the set of points whose distances to two foci have a constant difference.
Hypotenuse
The longest side of a right triangle, opposite the right angle.
Hypothesis
The "if" part of a conditional statement. In statistics, a claim that is tested using data.