Symbol
A symbol or (in many senses) token is a representation of something — an idea, object, concept, quality, etc..
A symbol can be a material object whose shape or origin is related, by nature or convention, to the thing it represents: for instance, the crucifix is the main symbol of Christianity, and the scepter is a traditional symbol of royal power.
It can also be a more or less conventional image (i.e. an icon), or a detail of an image, or even a pattern or color: for example, the olive branch in heraldry represents peace, the halo is a conventional symbol of sainthood in Christian imagery, tartans are symbols of Scottish clans, and the color red is often used as a symbol for socialist movements, especially communism.
More often, a symbol is a conventional written or printed sign (specifically, a glyph), usually standing for anything other than a sound (symbols for sounds are usually called graphemes, letters, logograms, diacritics, etc.). This Mathematical symbols such as π and + represent quantities and operations, currency symbols represent monetary units, and so forth.
Symbols can also be immaterial entities like words and gestures: in fact, the words of a language are symbols for concepts and relationships.
It is human beings' ability to manipulate symbols that allows them to explore the relationships between ideas, things, concepts, and qualities -- far beyond the explorations of which any other species on earth is capable. The discipline of semiotics studies symbols and symbol systems in general; semantics is specifically concerned with the main meaning of words.
Literary works are often admired for their artful use of symbolism. Religious and metaphysical writings are also known for their use of esoteric symbolism. The meaning of symbols is analysed by Freudian psychoanalysis, by Jungian analytical psychology and by literary semiotics.
The word "symbol" came to the English language, by way of Middle English, Old French, and latin, Latin, from the Greek root words συμ (sym) meaning "together" and βολοσ (bolos) from ballein meaning "to throw", having the approximate meaning of "to throw together".