Galactic Algorithms (2010)

In lay terms, an example of a galactic algorithm is one which has better big O complexity than algorithms currently in use, but where the crossover point of better performance than the status quo requires an incredibly large N - so large that we can't conceive any practical problem where this algorithm will perform better.

For example, you may need an N larger than the number of stars in the universe. Hence the term "galactic". I've seen another one that requires an N larger than the number of atoms in the universe.

A somewhat famous, more recent example: https://news.ycombinator.com/item?id=19474280

The Fastest and Shortest Algorithm for All Well-Defined Problems [1]

An algorithm M is described that solves any well-defined problem p as quickly as the fastest algorithm computing a solution to p, save for a factor of 5 and low-order additive terms.

(The constants in the "low order" terms are... large. Basically this is a delightful reductio of asymptotic analysis)

[1] https://arxiv.org/abs/cs/0206022

From the article: "A galactic algorithm is an algorithm that is wonderful in its asymptotic behavior, but is never used to actual compute anything."