× (multiplication sign)
The times sign, which means multiply. It is drawn differently from the letter x so the two are not confused; a raised dot · means the same thing.
X (Roman numeral)
The Roman numeral for 10. XX is 20, IX is 9 and XI is 11.
x (variable)
The letter most often used in algebra for an unknown number or a quantity that can change.
x-axis
The horizontal number line that passes through the origin on a coordinate plane.
x-axis symmetry
A graph has x-axis symmetry if folding it along the x-axis makes the top and bottom halves match: whenever (x, y) is on the graph, so is (x, −y).
x-bar
The symbol x̄, an x with a bar over it, which stands for the mean (average) of a sample of data.
x-coordinate
The first number in an ordered pair (x, y). It tells how far a point is to the left or right of the origin; also called the abscissa.
x-intercept
The point where a line or curve crosses the x-axis. There the y-value is always 0, so the point is written (a, 0).
xy-plane
The flat plane formed by the x-axis and the y-axis, used to graph points and equations; also called the coordinate plane.