A descriptive listing of words relating to periods of time is included below. Many of the words are adjectives in form, but also are commonly used as nouns, e.g., the bicentennial of the U.S. Constitution. All terms are defined in two separate lists: first by number referred to, then alphabetically.

diurnal, per diem, quotidian

daily; of a day

nocturnal

nightly; of a night

nichthemeron

a period of 24 hours

semidiurnal

twice a day

hebdomadal

weekly; a period of seven days

semiweekly

twice a week

biweekly

1. every two weeks

2. twice a week

fortnightly

once every two weeks

triweekly

1. every three weeks

2. three times a week

novendial

a period of nine days

monthly, tricenary

1. relating to a period of one month

2. thirty days

bimonthly

1. every two months

2. twice a month

semimonthly

twice a month

bimester

relating to a period of two months

trimester

relating to a period of three months

trimonthly

1. every three months

2. three times a month

biquarterly

twice every three months

biannual

twice a year (not necessarily at equally spaced intervals)

triannual

three times a year

semiannual, semiyearly, semestral

every half year or six-month period

annual, solennial, quotennial, per annum

yearly; once a year

biennial, biennium, biyearly, diennial

relating to a period of two years

triennial, triennium

relating to a period of three years

quadrennial, quadrennium, quadriennial

relating to a period of four years

quinquennial, quintennial, quinquennium

relating to a period of five years

sexennial, sextennial

relating to a period of six years

septenary, septennial, septennium

relating to a period of seven years

octennial

relating to a period of eight years

novennial

relating to a period of nine years

decennary, decennial, decennium

relating to a period of 10 years

undecennial

relating to a period of 11 years

duodecennial

relating to a period of 12 years

quindecennial

relating to a period of 15 years

septendecennial

relating to a period of 17 years

vicennial, vigintennial

relating to a period of 20 years

tricennial, trigintennial

relating to a period of 30 years

quinquagenary, semicentennial, semicentenary

relating to a period of 50 years

centenary, centennial, centennium, centurial

relating to a period of 100 years

quasquicentennial

relating to a period of 125 years

sesquicentenary, sesquicentennial

relating to a period of 150 years

bicentenary, bicentennial, bicentennium

relating to a period of 200 years

tercentenary, tricentennial, tercentennial

relating to a period of 300 years

quadricentennial, quatercentennial

relating to a period of 400 years

quincentenary, quincentennial

relating to a period of 500 years

sexcentenary

relating to a period of 600 years

septicentennial

relating to a period of 700 years

antemillennial, premillennial

relating to the period before the millennium

millennial, millennium

relating to a period of 1000 years; 10 centuries

postmillennial

relating to the period after the millennium

sesquimillennium

relating to a period of 1500 years; 15 centuries

bimillenary, bimillennial, bimillennium

relating to a period of 2000 years; 20 centuries

perennial

occurring year after year

plurennial

lasting for many years

aeonial

everlasting