It is broadly agreed that a nootropic is an ingestible substance (natural or artificial) that boosts one or more cognitive functions. The devil is in the details, however; check common sources for the term's exact definition and you'll see considerable variation [@wiktionary, @Merriam-webster, @medilexicon].

Although the terms "smart drug" and "nootropic" are often used interchangeably, there is an important distinction. As originally coined by Giurgea, nootropics work with few side effects and low toxicity (see "The nootropic concept and its prospective implications" [Giurgea, 1982 @Wiley]). Smart drugs like Ritalin or Adderall, have an acute effect on the nervous system with greater side effects and toxic load. The boundary is dangerously fuzzy, however.