I occasionally refer to reduplication in the word histories of my Good Words and Steve Parris has asked that I elaborate on it. Since it will require work on my part that I would prefer not to reduplicate (so to speak) in the future, I thought it better to share it with everyone.
Languages that use affixes (prefixes, suffixes, infixes) generally attach predefined affixes to the stems of words. In English, suffixes include -er, -ed, -ing, and the everpresent -s. They are spelled the same way—or vary slightly in predictable ways—when they are added to a stem: paint – paint-er, paint-ing, paint-s.
Some languages simply reduplicate the entire word as an affix. In Chinese, ren means “person” while renren means “everyone”. In Malay rumah means “house” while rumah-rumah means “houses”. Wiki in Hawaiian means “fast” while wiki-wiki means “very fast”, much like red, red rose means “a very red rose” in English.
Some languages, however, reduplicate prefixes, suffixes, and infixes from letters (or sounds) in each stem, so that they vary greatly from stem to stem. Ancient Greek, for example, had a prefix which we could symbolize as Ce-, where the capital C represents the first consonant of any Greek verb to which it is attached. This prefix marked the perfective aspect, indicating the absolute completion of the action named by the verb. In that language we find the following words:
bio- “live”: be-biō-k- (bebiōk-) “have lived”
game- “marry”: ge-gamē-k- (gegamēk-) “have married”
ly- “unfasten”: le-lŷk (lelŷk-) “have unfastened”
In other words, the initial consonant of this prefix “reduplicates” the first consonant of the stem to which it is attached. In the Tsimshian language, spoken by a native American people on the West Coast of the US, the plural is formed by adding the prefix Cik- where, again, the C will be whatever consonant occurs initially on the stem to which it is attached:
dasx “squirrel”: dik-dasx “squirrels”
seyp “bone”: sip-seyp “bones”
yexł “spit”: yip-yexł (yipyexł) “spits”
That is what “reduplication” is: copying a letter or two from the stem into a prefix or suffix that is added to it.