pentad

[pen-tad]
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.

Origin:
1645–55; < Greek pentad- (stem of pentás) group of five. See pent(a)-, -ad1

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pentad (ˈpɛntæd)
 
n
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
 
[C17: from Greek pentas group of five]

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