A sightline refers to a straight line or an imaginary line that connects two points, often used in reference to visual perspective. It is the path along which light rays travel from a source through space and reach our eyes. In other words, it's what we see when looking down a particular line of vision. Sightlines are commonly used by architects and designers while planning buildings or structures to ensure that they provide an unobstructed view for the people using them. For example, in a theatre, sight lines refer to the rows of seats that offer an unhindered view of the stage without any obstruction from other audience members.