The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018
Signifier: In linguistics, the “sound-image,” or word, used to represent a comparatively abstract concept, the signified. For example, the word flag is a signifier that calls up the idea of a flag, the signified, in the mind of an English-speaking person. According to Swiss linguist Ferdinand de Saussure, the relationship between signifier and signified is arbitrary — there is no intrinsic reason, for instance, why the word flag calls up the idea of a flag rather than a flea or a flood — and established by convention, such that the signifier and signified together constitute a sign understood by members of the same linguistic system. Indeed, one signifier may have many signifieds (eye may refer to the center of a hurricane, the hole in a needle, or the organ of sight), and one signified may have many signifiers (as is the case with synonyms such as big and large).
While Saussure privileged sound and speech in his analysis — set forth in the Cours de linguistique générale (Course in General Linguistics), published posthumously in 1916 and based on student notes of his lectures — contemporary theorists apply the term signifier equally to spoken and written forms of words. The term is also used in the field of semiotics (or semiology), the study of signs more generally, to refer to the material or physical form of a sign; for example, a thumbs-up gesture is a signifier of approval in some cultures, the color orange a signifier of construction.
Poststructuralists have pointed out that every signified is also a signifier, creating an endless chain of signification. For instance, while the signifier flag calls up the idea of a flag, the idea of a flag in turn evokes other signifieds, such as country and patriotism, which themselves evoke still more signifieds, such as courage and sacrifice.