The term signifier refers to any mark or sound, any perceivable indicator that communicates appropriate behavior to a person. Signifiers can be deliberate and intentional, such as the sign PUSH on a door, but they may also be accidental and unintentional, such as our use of the visible trail made by previous people walking through a field or over a snow-covered terrain to determine the best path.
Norman, Don. "The Psychopathology of Everyday Things." The Design of Everyday Things (n.d.): 1-37. Web.
<https://blackboard.stonybrook.edu/bbcswebdav/pid-3310347-dt-content-rid-20243251_1/courses/1154-CCS-395-SEC01-48098/Norman%20%2522POET%2522-OCR.pdf>.