What Is an Abscissa?

An abscissa is one of a pair of terms making up a set of coordinates in the Cartesian graphing system used in mathematics. This two dimensional system uses two perpendicular axes, one horizontal, called the x-axis, and one vertical, called the y-axis. Any point can be plotted using a set of two coordinates, called an ordered pair, with the x coordinate listed first and the y coordinate listed second. The x coordinate determines the horizontal position of the point in relation to the center of the graph, where the two axes meet, which is called the origin. The first number is the abscissa, while the second, or y value, is called the ordinate.

The term abscissa is derived from Latin, and is a form of the verb abscindo, meaning to cut off or tear away, but given the meaning of the Latin term, it is unclear how the word obtained this meaning. The first recorded use of the term in mathematics is believed to be in a mathematics text by the 17th century Roman mathematician, Stefano degli Angeli.

Cartesian coordinates for a point on a graph are written like this: (3,5) where the the first number of the pair is the x value of the point, and the second number is the y value. This means that in order to graph the point, one must move three units to the positive range of the horizontal, or x-axis, which is generally to the right of the origin. Then, starting from that point on the x-axis, one moves five units into the positive range of the vertical, or y-axis, which is generally upwards. A dot is placed at that point to denote the ordered pair, (3,5). The abscissa of that particular point is 3.

In some cases, particularly when used by physicists and astronomers, the term is used to refer to the entire x-axis, rather than a particular point on it. This rarely creates confusion, however, as the context for this usage is different enough that those with knowledge of these fields are able to discern the speaker or writer's intent. In some older writings on mathematics and geometry, the term is used in yet another way. In equations of the form a=bt, which describes a line in Euclidean geometry, the abscissa, denoted in the equation as the t value, indicates a point on the line a given distance t from the origin.