pentad

noun

1. a period of five years.
2. a group of five.
3. the number five.
4. Chemistry. a pentavalent element or group.
5. Climatology. a period of five consecutive days.

noun

1. a group or series of five
2. the number or sum of five
3. a period of five years
4. chem a pentavalent element, atom, or radical
5. meteorol a period of five days

n.1650s, from Greek pentas (genitive pentados) “group of five” (see five). Meaning “period of five years” is from 1880; meaning “period of five days” is from 1906, originally in meteorology.