A checkbox is a small square on a paper form or a computer screen. You put a mark in it to choose that option — a tick, an x, or a click. You will see checkboxes on tests ('tick all that apply'), on websites (agree to the terms), and in settings. The box is empty until you mark it. On a computer, one click usually puts the mark in and another click takes it out. It is a normal, neutral word in both American and British English, though British speakers also say tick box. Though actually, the checkbox is the square itself, not the mark inside it.