This is currently under construction but will eventually be a page for the basic (and not so basic) definition of improv terms. As this site develops I would like it to be so very full of information that someone could feel confident starting their own troupe because they have this resource.
Shortform- The definition we all cling to when describing short form to potential audiences is "Like Whose Line is it Anyway?" Which is perhaps the greatest thing that show has brought to improv as an art form. Shortform is fast, punchy games that generally last between 1 to 5 minutes.
Longform- Often described as an "Improvised Play", longform generally has less strict games compared to shortform, but an overall structure. Longforms are, on average, 20-30 minutes but can be as short as 15 or so and as long as a full-length play. Arguably the most famous long form is the Harold.
Midform- Midform is the bastard child of longform and shortform. Something described as a midform is usually something with a more set game (stay in one location, all of your suggestions are made up words from a scrabble game, big blind) but tend to have one or two things that lack compared to an all out longform. Not everyone acknowledges midform as a separate entity, describing midform games as long short form games or short longforms.
Game of a scene-The Game is a phrase improvisers use for the thing going on in a scene. A scene, character, or entire form can have a game. The most simple example of a game is a repetitive reaction to something (every time ___ does ___, I do __).
Group Game- often a break away from actual scenework to something with the main intent of populating ideas for the rest of the scenes in a long form (ex. Hot Spot, The Endowment, a Debate)
The Harold- Described fully in Truth in Comedy and Art by Committee (both books produced by Charna Halpern of the iO in Chicago), the Harold is a very important longform which uses a structure of Group Game-three scenes- Group Game- three iterations* of the original scenes-Group Game-three further iterations (Optional concluding group game).
*Iterations refers to any connective feature to those original scenes be it a time lapse, use of the same location, thematic, same characters, really any form of connection. The Harold is so versatile that different formats, group games, constructs, and rules have been added and taken away to make an entire field of harold based forms.