Tablespoon
A unit of volume used in cooking, equal to 3 teaspoons or about 15 mL. It is abbreviated tbsp or T.
Tally marks
Short lines used to keep count, grouped in fives with a diagonal line across every fifth mark.
Tangent (of a circle)
A line that touches a circle at exactly one point. It is perpendicular to the radius at that point.
Tangent (trigonometry)
In a right triangle, the ratio of the side opposite an angle to the side adjacent to it: tan θ = opposite / adjacent.
Tangram
A Chinese puzzle of seven flat pieces cut from a square, which can be rearranged to make many shapes.
Tape diagram
A drawing of rectangular strips used to show ratios or the parts of a word problem; also called a bar model.
Tax
Money that people or businesses pay to the government, often a percent of a price or income.
Tax rate
The percent of a price or income charged as tax.
Teaspoon
A small unit of volume used in cooking, about 5 mL. It is abbreviated tsp or t.
Tenth
One of ten equal parts of a whole, written 1/10 or 0.1. It is the first place to the right of the decimal point.
Term
A single number, variable, or product of numbers and variables in an expression; also, one number in a sequence.
Terminal side
The ray where an angle in standard position ends after rotating from the initial side on the positive x-axis.
Terminating decimal
A decimal that ends after a certain number of digits, such as 0.375.
Tessellation
A pattern of shapes that covers a flat surface with no gaps or overlaps; also called a tiling.
Tetrahedron
A polyhedron with four triangular faces; a triangular pyramid.
Theorem
A statement that has been proven true using definitions, postulates and other theorems.
Theoretical probability
The probability an event will happen based on reasoning: the number of favorable outcomes divided by the number of possible outcomes.
Thousand
The number 1,000, equal to 10 hundreds or 10³.
Thousandth
One of 1,000 equal parts of a whole, written 1/1000 or 0.001. It is the third place to the right of the decimal point.
Three-dimensional
Having length, width and height, like a solid figure; often written 3-D.
Tip
Extra money given to someone for a service, such as a server in a restaurant, often a percent of the bill.
Tip rate
The percent of the bill paid as a tip.
Ton
A unit of weight. A US (short) ton is 2,000 pounds; a metric ton (tonne) is 1,000 kilograms.
Total
The whole amount found by adding all the parts together.
Transformation
A change in the position, size or shape of a figure, such as a translation, reflection, rotation or dilation.
Transitive property
If a = b and b = c, then a = c. It also works for inequalities: if a > b and b > c, then a > c.
Translation
A transformation that slides every point of a figure the same distance in the same direction.
Transversal
A line that crosses two or more other lines at different points.
Trapezoid
A quadrilateral with exactly one pair of parallel sides, called the bases. (Some books say at least one pair.)
Tree diagram
A diagram with branches that shows all the possible outcomes of an experiment.
Triangle
A polygon with three sides and three angles. Its angles always add up to 180°.
Triangle inequality theorem
The sum of any two sides of a triangle must be greater than the third side.
Triangular number
A number of dots that can form an equilateral triangle: 1, 3, 6, 10, …
Triangular prism
A prism with two parallel triangular bases joined by three rectangular faces.
Trigonometry
The branch of mathematics that studies the relationships between the sides and angles of triangles, using sine, cosine and tangent.
Trillion
One thousand billion: 1,000,000,000,000, or 10¹².
Trinomial
A polynomial with exactly three terms, such as x² + 5x + 6.
Truncate
To shorten a number by cutting off digits after a certain place, without rounding.
Twin primes
Pairs of prime numbers that differ by 2, such as 3 and 5, 11 and 13, or 17 and 19.
Two-dimensional
Having length and width but no height, like a flat shape; often written 2-D.
Two-step equation
An equation that takes two inverse operations to solve, such as 2x + 3 = 11.
Two-way table
A table that shows data for two categorical variables at once, with totals for the rows and columns.