The best nootropic is getting a good nights sleep. It takes effort! Preparing for bed early and disciplining yourself to put the light out is not easy. Not nearly as easy as popping a pill. But it's so much more effective than any drug listed here. And for a safe and provable quick cognitive boost, take some caffeine, preferable green tea with theanine.
Here's a concise definition[0]:
"(of a drug) used to enhance memory or other cognitive functions"
There are other definitions. All the ones I've seen have exogenous substances as a common denominator, however.