A Decade

1. A group, set, or series of ten [from 16th c.], particularly:

1. 1. A period of ten years [from 17th c.], particularly such a period beginning with a year ending in 0 and ending with a year ending in 9. [from 19th c.] Synonym: (in some contexts) decennium
   2. A period of ten days, (history) particularly those in the ancient Egyptian, Coptic, and French Revolutionary calendars. [from 18th c.]
   3. (literary, archaic) A work in ten parts or books, particularly such divisions of Livy's History of Rome. [from 15th c.]
   4. (Roman Catholicism) A series of prayers counted on a rosary, typically consisting of an Our Father, followed by ten Hail Marys, and concluding with a Glory Be and sometimes the Fatima Prayer.
   5. Any of the sets of ten sequential braille characters with predictable patterns.
   6. (electronics) A set of ten electronic devices used to represent digits.
2. (electronics) A set of resistors, capacitors, etc. connected so as to provide even increments between one and ten times a base electrical resistance.
3. (physics, engineering) The interval between any two quantities having a ratio of 10 to 1.

Usage notes

Although a decade may refer to any group of ten years, it often particularly refers to the informal ten-year periods of the calendar whose last digits run from 0 to 9. Some style guides may prefer that decade refers exclusively to such calendar periods while decennium, decennary, &c. refers to ten-year periods in other contexts.

It should be noted that the method of computing a decade is distinguished from the proper computation of centuries and millennia, which run from 1 to 0. The 1st century began with the year 1 and ended with the year 100, but "the Nineties" are the years whose name includes the word ninety, from '90 to '99 with all those years with a 9 in the tens place digit.