Recursion

Recursion is a holistic solve path element in which a particular extraction mechanism or gimmick in a puzzle is used both to extract something, only to be applied in some way to the extracted information again, in a way providing an extraction within an extraction. Can also be used to refer to puzzles that involve true recursion, such as recursive mazes, that may have an infinite number of themselves within them.

Background
Recursion as a concept pertaining to puzzle hunts is obviously much newer than the linguistic, mathematical, or computer science versions. Even then, puzzles have referenced these types of recursions, and they have certainly influenced the hunt definition of it.

Non-Puzzle Background
See also: Recursion

Traditionally, recursion occurs when a process involves running that same process as one of its steps,

Puzzle Background
Similarly, puzzle-based recursion involves the repetition of steps already taken in order to achieve the final result. However in most cases puzzles that "utilize" the recursion element are not themselves fully recursive. This is due to the difficulty of creating a puzzle that both doesn't end and provides a satisfying conclusion. Instead, the definition was broadened to include cases of individual steps (particularly extraction steps) being used after they've already changed the state of a puzzle.

Puzzle Application
Puzzles that are recursive vary in the level of recursion that they involve, as it can vary greatly between puzzles. A puzzle may only repeat a theme that did not otherwise get used to extract letters or other final information; it may repeat an extraction method or common transformation, applying them to a final cluephrase or set of information; or it may be truly recursive, with the puzzle or an aspect of the puzzle repeating infinitely (or at least appear to).

Theme Repetition
Sometimes, a puzzle with an overarching theme will result in a final cluephrase that, at first glance, doesn't provide enough information to be able to solve it. In fact, these kinds of recursive puzzles will often choose a cluephrase that cannot be solved in a vacuum. Instead, solvers need to apply the theme or context of a puzzle to whatever has been extracted in order to get something useful. Theme repetition is usually found at the end of a puzzle, as any puzzle with an overarching theme can only revisit that theme once it appears to be no longer useful.

For example: A puzzle involving winners of the Grammy award for Best New Album solves to the phrase "NINETY SIX". This is NOT the final answer. Interpreting it as "The 96th Grammy Awards" doesn't work (there haven't been that many), but interpreting it as "Winner in 1996" results in the final answer JAGGED LITTLE PILL.

Gimmick Repetition
Perhaps the most common type of puzzle recursion, gimmick repetition is exactly as it sounds. A puzzle may use a gimmick to extract information or otherwise progress the puzzle to a new state, such as providing clues that need to be solved a particular (non-straight) way, or transforming answers in order to place them in a grid. If these gimmicks are then reused to solve a final cluephrase, transform an incorrect answer into a correct one, or are otherwise applied in the same way to a state of a puzzle that has already had the gimmick applied to it, then that's recursion. This is also the main premise behind Tortured Clues puzzles.

For example: A puzzle involving finding odd-ones-out results in five rows of five "odd" words, one of which being SALMON, TOMATO, SCARLET, HARLEQUIN, and CINNABAR. Repeating the process of finding the odd one out extracts HARLEQUIN from this set, as it's a shade of green while the others are shades of red. Presumably, the other rows can be solved in similar ways. Note: This example could go one more layer further if the extracted words form another set with one odd-word-out.

True Recursion
True recursion is rare in the grand scheme of puzzles, as it tends to require a more rigorous planning to make it both function correctly and have a way to resolve in the way expected of a hunt puzzle. As such, truly recursive puzzles are usually paired with topics like geometry or computer science due to their relationship with recursion as a concept. There are outliers, of course, but even then they will sometimes utilize methods from mathematics and computer science in order to properly execute multiple layers of recursion, particularly if they're attempting to produce infinite-or-close-to-it recursion.

For example: A puzzle presents as an interactive escape room, in which a smaller model of the escape room (and themselves) has been placed, allowing the player to interact with the room they're in on a larger or smaller scale.

Notable Examples

 * Backlot (MITMH 2020) - True Recursion. A maze/navigation logic puzzle that contains smaller versions of the same puzzle that must be solved before continuing along the rest of the path. Goes three levels of recursion deep (not including the base maze) at maximum during the final solution.