A vertical line is a straight line that runs up and down, parallel to the y-axis in a Cartesian coordinate system. It has an undefined slope and can be represented by an equation of the form x = a, where "a" is a constant value. This means that for any point on the line, the x-coordinate remains the same while the y-coordinate can vary. In geometry, vertical lines are often used to represent boundaries or divisions. They can be found in various contexts, such as in architecture, where they help define the height of structures, or in graphs, where they can indicate specific values or limits.