Definition: Semiology is the analysis of communication systems by the study of signs. Ferdinand de Saussure defined in 1915: “ We can conceive of a science which studies the life of signs within social life (…); we will call it semiologies ».
A communication system is not limited to conventional signs such as the highway code, it is with semiology to consider that all communication also resides in the relationship between, on the one hand, a material plane, that of the expression, "the signifier", and, on the other, a conceptual level, that of content, "the signified". For example for the word "tree", the sequence of the five letters is the signifier while the signified is the mental representation of the tree that you will make for yourself.
In communication, semiology is used to determine the positioning of a brand (brand narrative, discourse platform, expressions of identity, etc.).