To make marks to represent objects counted, and record these marks in a table (tally chart).

1. A line that touches a curve at one point only.

2. A trigonometric function (more on these at GCSE level).

A solid with four triangular faces. A regular tetrahedron has faces that are equilateral triangles. Plural: tetrahedra.

A mathematical statement derived from previously accepted premises and established by means of a proof. A 'theory' is slightly different - this is a testable model that can be used to make predictions, but isn't yet proven.

In geometry, a type of transformation in which every point of a shape moves the same distance in the same direction; a transformation that is  specified by a distance and direction (vector).

A quadrilateral with exactly one pair of sides parallel.

A branching, decision diagram in which probabilities may be assigned to each branch and used to determine the probability of any outcome of combined or compound events.

A polygon with three sides. Adjective: triangular, having the form of a triangle.