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This biohazard sign is a completely conventional symbol with no inherent relationship to what it represents.

This "no smoking" symbol combines the conventional "no" symbol with a somewhat abstracted image of a cigarette.

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)

The way in which a sign signifies is a topic in philosophy of language, see also Meaning (linguistic)

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:

1. There are things that are just things, not any sign at all;
2. There are things that are also signs of other things (as natural signs of the physical world and mental signs of the mind);
3. There are things that are always signs, as languages (natural and artificial) and other cultural nonverbal symbols, as documents, money, ceremonies, and rites.

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:

A signboard on a beach in Durban in apartheid-era South Africa indicates a racially segregated beach.

• Sign, in astrology, often used to mean the Sun sign
• Sign or signing, in communication, refers to communicating via hand gestures, such as sign language.
• Gang signal
• A signboard.
• Signedness, in computing, the digits where one bit among them denotes whether the number is either negative or non-negative, are called signed, otherwise unsigned. See also signed number representation
• Sign, in common use, refers to an indication that a previously observed event is about to occur again
• Sign, in divination and religion, an omen, an event or occurrence believed to foretell the future
• Sign, in ontology and spirituality, a coincidence; see synchronicity
• Sign (linguistics), a combination of a concept and a sound-image described by Ferdinand de Saussure
• Sign, in mathematics, whether a number is negative or positive
• Sign, in biology, an indication of some living thing's presence
• Medical sign, in medicine, objective evidence of the presence of a disease or disorder, as opposed to a symptom, which is subjective
• Sign (semiotics), the basic units of meaning
• Information sign, a notice that instructs, advises, informs or warns people
• Traffic sign, signs that instruct drivers; see also stop signs, speed limit signs, cross walk signs
• Sign, in a writing system, a basic unit. Similar terms which are more specific are character, letter or grapheme
• Commercial signage, including flashing signs, such as on a retail store, factory, or theatre
• Signature, in history, a handwritten depiction observed on a document to show authorship and will

Wikimedia Commons has media related to:

signs

• Charles Sanders Peirce
• Claude Levi-Strauss
• Divination
• Edmund Leach
• Ferdinand de Saussure
• Icon
• Icon (computing)
• Ideogram
• Interpretation of dreams
• List of symbols
• Logotype
• Map-territory relation
• Mary Douglas
• National symbol
• Religious symbolism
• Representation
• Roland Barthes
• Semiotics
• Signing
• Structuralism
• Symbol
• Synchronicity
• Traffic sign