"1 more than ninety-nine, ten times ten; the number which is one more than ninety-nine; a symbol representing this number;" Old English hundred "the number of 100, a counting of 100," from Proto-Germanic *hunda-ratha- (source also of Old Frisian hundred, Old Saxon hunderod, Old Norse hundrað, German hundert); first element is Proto-Germanic *hundam "hundred" (cognate with Gothic hund, Old High German hunt), from PIE *km-tom "hundred," reduced from *dkm-tom- (source also of Sanskrit satam, Avestan satem, Greek hekaton, Latin centum, Lithuanian šimtas, Old Church Slavonic suto, Old Irish cet, Breton kant "hundred"), suffixed form of root *dekm- "ten."

The second element is Proto-Germanic *rath "reckoning, number" (as in Gothic raþjo "a reckoning, account, number," garaþjan "to count;" from PIE root *re- "to reason, count"). The common word for the number in Old English was simple hund, and Old English also used hund-teontig.

In Old Norse hundrath meant 120, that is the long hundred of six score, and at a later date, when both the six-score hundred and the five-score hundred were in use, the old or long hundred was styled hundrath tolf-roett ... meaning "duodecimal hundred," and the new or short hundred was called hundrath ti-rætt, meaning "decimal hundred." "The Long Hundred and its use in England" was discussed by Mr W.H. Stevenson, in 1889, in the Archæological Review (iv. 313-27), where he stated that amongst the Teutons, who longest preserved their native customs unimpaired by the influence of Latin Christianity, the hundred was generally the six-score hundred. The short hundred was introduced among the Northmen in the train of Christianity. [Transactions of the Lancashire and Cheshire Antiquarian Society, 1907]

Meaning "division of a county or shire with its own court" (still in some British place names and U.S. state of Delaware) was in Old English and probably represents 100 hides of land. The Hundred Years War (which ran intermittently from 1337 to 1453) was first so called in 1874. The original Hundred Days was the period between Napoleon's restoration and his final abdication in 1815.

"consisting of or lasting 100 years, happening every 100 years," 1789, from Latin centum "one hundred" (see hundred) + ending from biennial. As a noun, "a hundredth anniversary celebration," from 1876; the older noun is centenary.

"pertaining to a 200-year period," 1843; see bi- + centenary. Also see bicentennial. As a noun, "two-hundredth anniversary or celebration," from 1840.

1805, "person 100 years old or older," from centenary + -ian. As an adjective, "pertaining to a person 100 years old," it is recorded from 1806.

Proto-Indo-European root meaning "ten."

It forms all or part of: cent; centenarian; centenary; centi-; centime; centurion; century; centennial; cinquecento; dean; deca-; decade; decagon; Decalogue; Decameron; decapod; decathlon; December; decennial; deci-; decile; decimal; decimate; decimation; decuple; decussate; denarius; denier (n.) "French coin;" dicker; dime; dinar; doyen; dozen; duodecimal; duodecimo; eighteen; fifteen; fourteen; hecatomb; hendeca-; hundred; icosahedron; nineteen; nonagenarian; octogenarian; Pentecost; percent; quattrocento; Septuagint; sexagenarian; seventeen; sixteen; ten; tenth; thirteen; thousand; tithe.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."

1868, "a 500th anniversary;" 1874 as an adjective, "relating to or consisting of 500 years," irregularly formed from Latin quinque "five" (see quinque-) + centenarius "consisting of a hundred" (see centenary). A better formation is quingenary, from Late Latin quingenarius "consisting of five hundred each," from quingeni, distributive of quingenti, quincenti "five hundred." Quingentenary (1872) also was tried.

"a unit of weight equal to a hundred pounds," c. 1400, quintale (mid-14c. in Anglo-French), from Old French quintal "hundredweight," and directly from Medieval Latin quintale, from Arabic quintar, from Late Greek kentenarion, from Latin centenarius "containing a hundred" (see centenary).

1832, "pertaining to a period of 300 years," from ter- "three times" + centenary. As a noun from 1835. Related: Tercentennial (1862 as an adjective; 1872 as a noun).