In microbiology, a Gram-negative bacterial endosymbiont that is found in the cytoplasm of species of the aquatic ciliate Euplotes (a genus of ciliate protozoa having a dorsoventrally flattened body with widely spaced rows of short bristle-like cilia, simple microscopic organism with projecting threads that thrash to help it to move along, on the dorsal surface).

1. The fifteenth letter and fifth vowel of the Greek alphabet (Ο, ο), represented in the English alphabet as short "o".
2. As a numeral, it denotes 70.
3. Etymology: Middle English, from Greek o mikron, small or little "o" (because of its being a short vowel in Greek); as opposed to omega, "big o, long o".