Improvisation, often shortened to improv, is the activity of making or doing something not planned beforehand, using whatever can be found. The origin of the word itself is in the Latin "improvisus", which literally means un-foreseen. Improvisation in the performing arts is a very spontaneous performance without specific or scripted preparation. The skills of improvisation can apply to many different faculties across all artistic, scientific, physical, cognitive, academic, and non-academic disciplines; see Applied improvisation.

The skills of improvisation can apply to many different abilities or forms of communication and expression across all artistic, scientific, physical, cognitive, academic, and non-academic disciplines. For example, improvisation can make a significant contribution in music, dance, cooking, presenting a speech, sales, personal or romantic relationships, sports, flower arranging, martial arts, psychotherapy, and much more.

Techniques of improvisation are widely used in training for performing arts or entertainment; for example, music, theatre and dance. To "extemporize" or "ad lib" is basically the same as improvising. Colloquial terms such as "playing by ear", "take it as it comes", and "making it up as [one] goes along" are all used to describe improvisation.

The simple act of speaking requires a good deal of improvisation because the mind is addressing its own thought and creating its unrehearsed delivery in words, sounds and gestures, forming unpredictable statements that further feed the thought process (the performer as the listener), creating an enriched process that is not unlike instantaneous composition with a given set or repertoire of elements.

Where the improvisation is intended to solve a problem on a temporary basis, the "proper" solution being unavailable at the time, it may be known as a "stop-gap". This applies to the field of engineering. Another improvisational, group problem-solving technique being used in organizations of all kinds is brainstorming, in which any and all ideas that a group member may have are permitted and encouraged to be expressed, regardless of actual practicality. As in all improvisation, the process of brainstorming opens up the minds of the people involved to new, unexpected and possibly useful ideas. The colloquial term for this is "thinking outside the box."

Improvisation can be thought of as an "on the spot" or "off the cuff" spontaneous moment of sudden inventiveness that can just come to mind, body and spirit as an inspiration. Viola Spolin created theater games as a method of training improvisational acting. Her son, Paul Sills popularized improvisational theater, or IMPROV, by using Spolin's techniques to train The Second City in Chicago, the first totally improvisational theater company in the US.

Musical improvisation is usually defined as the spontaneous performance of music without previous preparation or any written notes. In other words, the art of improvisation can be understood as composing music "on the fly". There have been experiments by Charles Limb, using functional magnetic resonance imaging, that show the brain activity during musical improvisation. Limb showed increased activity in the medial prefrontal cortex, which is an area associated with an increase in self-expression. Further, there was decreased activity in the lateral prefrontal cortex, which is an area associated with self-monitoring. This change in activity is thought to reduce the inhibitions that normally prevent individuals from taking risks and improvising.

Notable improvisational musicians from the modern era include Keith Jarrett, an improvisational jazz pianist and multi-instrumentalist who has performed many improvised concerts all over the world; W. A. Mathieu a.k.a. William Allaudin Mathieu, the musical director for The Second City in Chicago, the first ongoing improvisational theatre troupe in the United States, and later musical director for another improv theatre, The Committee, an offshoot of The Second City in San Francisco; Derek Bailey, an improvisational guitarist and writer of Improvisation: Its Nature and Practice; Evan Parker; British saxophone player, the iconnical pianists Fred van Hove (Be) and Misha Mengelberg (NL) and more recently the Belgian Seppe Gebruers who improvise with two pianos tuned a quartertone apart.