在一种认知体系中，符号是指代一定意义的意象，可以是图形图像、文本组合，也可以是声音信号、建筑造型，甚至可以是一种思想文化、一个时事人物。例如“=”在数学中是等价的符号，“紫禁城”在政治上是中国古代皇权的象征。总的来说，符号的功能是携带和传达意义。它具有极为广泛的含义，来自于规定或约定俗成。

以传播学的视角来看，麦克卢汉在《理解媒介》一书中提出，媒介即讯息。信息是符号和意义的统一体，符号是信息的外在形式或物质载体。结构语言学家索绪尔在《普通语言学教程》中界定为能指和所指。英国学者特伦斯·霍克斯认为“任何事物只要它独立存在，并和另一事物有联系，而且可以被‘解释’，那幺它的功能就是符号。”

A sign is an object, quality, event, or entity whose presence or occurrence indicates the probable presence or occurrence of something else. A natural sign bears a causal relation to its object—for instance, thunder is a sign of storm, or medical symptoms signify a disease. A conventional sign signifies by agreement, as a full stop signifies the end of a sentence; similarly the words and expressions of a language, as well as bodily gestures, can be regarded as signs, expressing particular meanings. The physical objects most commonly referred to as signs (notices, road signs, etc., collectively known as signage) generally inform or instruct using written text, symbols, pictures or a combination of these.