Reduplication is repeating a syllable with a slight change to the vowel or to a consonant. Reduplication is how you make the present and past perfect tenses in Greek, so you probably know the word already if you studied Greek. 

We have it in English, too, but we don’t mention it (except maybe in linguistic circles). We use reduplication mainly in onomatopoetic words: tic tock, clip clop, and so on. 

Why do I mention this? I ran into an article that mentions reduplication that also mentions a topic I wrote about two or so years ago but didn’t know the source, so I’m giving credit now. 

(I wrote about ablaut a while back, too, but in a different context. In case you’re interested, it’s here. Ablaut is when you change a vowel following a pattern.)

Here’s a paragraph from the article:

You are utterly familiar with the rule of ablaut reduplication. You’ve been using it all your life. It’s just that you’ve never heard of it. But if somebody said the words zag-zig, or ‘cross-criss you would know, deep down in your loins, that they were breaking a sacred rule of language. You just wouldn’t know which one.

It’s an interesting article; I recommend you read the whole thing.

PS—sigh. Wouldn’t you know, today I ran into a comic that features ablaut reduplication.

PPS—and here’s a serious use of the word:

The name comes from Hawaiian ʻoumuamua, meaning ‘scout’ (from ʻou, meaning ‘reach out for’, and mua, reduplicated for emphasis, meaning ‘first, in advance of’), and reflects the way this object is like a scout or messenger sent from the distant past to reach out to humanity. (That apostrophe at the beginning of the name is a glottal stop, not an orphaned single quote.)

JJB [astrophysig]