A sign is an entity which signifies another entity. A natural sign is an entity which bears a causal relation to the signified entity, as thunder is a sign of storm. A conventional sign signifies by agreement, as a full stop signifies the end of a sentence. (Contrast a symbol which stands for another thing, as a flag may be a symbol of a nation)
Any given signifier or symbol is dependent upon that which is intended, expressed, or signified in a semiotic relationship of signification, significance, meaning, or import. Thus, for example, people may speak of the significance of events, the signification of characters, the meaning of sentences, or the import of a communication. These different relationships that exist between sorts of signs can help people and sorts of things that are signified can be called the modes of signification.
The range of uses of signs are varied. They might include: the indication or mark of something, a display of a message, a signal to draw attention, evidence of an underlying cause (for instance, the symptoms of a disease are signs of the disease), a character for a mathematical operation, a body gesture, etc.
Semiotics, epistemology, logic, and philosophy of language are concerned about the nature of signs, what they are and how they signify. The nature of signs and symbols and significations, their definition, elements, and types, is mainly established by Aristotle, Augustine, and Aquinas. According to these classic sources, significance is a relationship between two sorts of things: signs and the kinds of things they signify (intend, express or mean), where one term necessarily causes something else to come to the mind. Distinguishing natural signs and conventional signs, the traditional theory of signs sets the following threefold partition of things:
Thus there are things which may act as signs without any respect to the human agent (the things of the external world, all sorts of indications, evidences, symptoms, and physical signals), there are signs which are always signs (the entities of the mind as ideas and images, thoughts and feelings, constructs and intentions); and there are signs that have to get their signification (as linguistic entities and cultural symbols). So, while natural signs serve as the source of signification, the human mind is the agency through which signs signify naturally occurring things, such as objects, states, qualities, quantities, events, processes, or relationships. Human language and discourse, communication, philosophy, science, logic, mathematics, poetry, theology, and religion are only some of fields of human study and activity where grasping the nature of signs and symbols and patterns of signification may have a decisive value.
A sign can denote any of the following: