Stimulants (also referred to as psychostimulants) are psychoactive drugs that induce temporary improvements in either mental or physical functions or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others. Due to their rendering a characteristic "up" feeling, stimulants are also occasionally referred to as "uppers". Depressants or "downers", which decrease mental and/or physical function, are in stark contrast to stimulants and are considered to be the functionally opposite drug class. Stimulants are widely used throughout the world as prescription medicines and without prescription both as legal substances and illicit substances of recreational use or abuse.