A checkbox (check box, tickbox, tick box) is a GUI widget that permits the user to make a binary choice, i.e. a choice between one of two possible mutually exclusive options. For example, the user may have to answer 'yes' (checked) or 'no' (not checked) on a simple yes/no question.
Checkboxes are often shown on the screen as a square box that can contain white space (for false) or a tick mark or × (for true), as shown ( ☐ or ☑, respectively). A caption describing the meaning of the checkbox is normally shown adjacent to the checkbox. Inverting the state of a checkbox is done by clicking the mouse on the box, or the caption, or by using a keyboard shortcut, such as the space bar.
Often, a series of checkboxes are presented, each with a binary choice between two options. Then the user may select several of the choices. Compare this to a radio button, in which only a single option is selectable from several mutually-exclusive choices.
Some applications use checkboxes that allow an indeterminate state in addition to the two provided by a normal checkbox. This third state is shown as a square or dash in the checkbox, and indicates that its state is neither checked nor unchecked. This is most often used when the checkbox is tied to a collection of items in mixed states. The indeterminate state cannot usually be selected by the user, and switches to a checked state when activated.