Fashion is a form of self-expression and autonomy at a particular period and place and in a specific context, of clothing, footwear, lifestyle, accessories, makeup, hairstyle, and body posture. In its everyday use, the term implies a look defined by the fashion industry as that which is trending. Everything that is considered fashion is available and popular by the fashion system (industry and media).
Fashion is a form of expression. Fashion is what people wear in a specific context. If a stranger would appear in this setting, adorning something different, the stranger would be considered "out of fashion.