A tell-tale (or idiot light, especially in North America) is an indicator of the status or malfunction of a system within a motor vehicle. Tell-tales consist of a light bulb or an LED which lights up a symbol or text legend. The "idiot light" terminology arises from popular frustration with automakers' use of tell-tales for crucial functions which could previously be monitored by gauges so a troublesome condition could be detected and corrected early. Such early detection of problems with, for example, engine temperature or oil pressure or charging system operation is not possible via a tell-tale, which usually illuminates only once a fault has already occurred—thus providing no advance warning or details of the malfunction's extent. The Hudson automobile company was the first to use lights instead of gauges for oil pressure and the voltmeter, starting in the mid-1930s.

Automotive tell-tales are regulated by automobile safety standards worldwide. In the United States, National Highway Traffic Safety Administration Federal Motor Vehicle Safety Standard 101 includes tell-tales in its specifications for vehicle controls and displays. In Canada, the analogous Canada Motor Vehicle Safety Standard 101 applies. In Europe and throughout most of the rest of the world, ECE Regulations specify various types of tell-tales.

An idiot, dolt, or dullard is a mentally deficient person, or someone who acts in a self-defeating or significantly counterproductive way. Archaically the word mome has also been used. The similar terms moron, imbecile, and cretin have all gained specialized meanings in modern times. An idiot is said to be idiotic, and to suffer from idiocy. A dunce is an idiot who is specifically incapable of learning. An idiot differs from a fool (who is unwise) and an ignoramus (who is uneducated/an ignorant), neither of which refers to someone with low intelligence.

Idiot as a word derived from the Greek ἰδιώτης, idiōtēs ("person lacking professional skill", "a private citizen", "individual"), from ἴδιος, idios ("private", "one's own"). In Latin the word idiota ("ordinary person, layman") preceded the Late Latin meaning "uneducated or ignorant person." Its modern meaning and form dates back to Middle English around the year 1300, from the Old French idiote ("uneducated or ignorant person"). The related word idiocy dates to 1487 and may have been analogously modeled on the words prophet and prophecy. The word has cognates in many other languages.