The incorrect plural of the word 'octopus', using a Latin ending for a word ending in 'us', by replacing it with an 'i', as in 'octopi'.
In English, it is correct to say 'octopuses', first used by uneducated peasants who did not know of the word's Greek origin, however, it is still technically incorrect.
The word 'octopus' is Greek (octo=eight, pus=foot), therefor, it's ending should be a Greek ending. The Greek ending of the word 'octopus' is 'octopodes' (pronounced: Ock-Top-Oh-Dez).
Octopi is the number, eight multiplied by pi, so is 25.13274123...