重叠词是包含两个相同或非常相似部分的单词或词素（如mama）。像这样的词也被称为重言词。通过重复一个复合词的全部或部分而形成复合词的形态和语音过程称为重叠。重复的元素称为重叠元素。

David Crystal在《剑桥英语百科全书》第二版中写道：

"Items with identical spoken constituents, such as goody-goody and din-din, are rare. What is normal is for a single vowel or consonant to change between the first constituent and the second, such as see-saw and walkie-talkie. "Reduplicatives are used in a variety of ways. Some simply imitate sounds: ding-dong, bow-wow. Some suggest alternative movements: flip-flop, ping-pong. Some are disparaging: ​dilly-dally, wishy-washy. And some intensify meaning: teeny-weeny, tip-top. Reduplication is not a major means of creating lexemes in English, but it is perhaps the most unusual one." (Cambridge Univ. Press, 2003)

特点

重叠词可以押韵，但不要求押韵。它们很可能有一个声音的图形，因为头韵（辅音重复）和协和音（元音重复）在一个单词或短语中很常见，但在各个部分之间变化不大，如Patrick B.Oliphant的这篇文章，“如果我错了，请纠正我：gizmo连接到Flinglang连接到watzis，watzis连接到doo dad连接到叮当。”

根据Kate Burridge的“Gob的礼物：英语语言历史片段”：

"The majority of...reduplicated forms involve a play on the rhyme of words. The result can be a combination of two existing words, like flower-power and culture-vulture, but more usually one of the elements is meaningless, as in superduper, or both, as in namby-pamby. Now, it struck me the other day that a large number of these nonsense jingles begin with 'h.' Think of hoity-toity, higgledy-piggledy, hanky-panky, hokey-pokey, hob-nob, heebie-jeebies, hocus-pocus, hugger-mugger, hurly-burly, hodge-podge, hurdy-gurdy, hubbub, hullabaloo, harumscarum, helter-skelter, hurry-scurry, hooley-dooley and don't forget Humpty Dumpty. And these are just a few!" (HarperCollins Australia, 2011)

重叠词与回声词的不同之处在于，重叠词的形成规则较少。

借用重叠词

英语重叠词的历史始于15世纪末的早期现代英语时代。在《英语传记》第三版中，C.M.Millward和Mary Hayes指出：

"Reduplicated words do not appear at all until the EMnE period. When they do appear, they are usually direct borrowings from some other language, such as Portuguese dodo (1628), Spanish grugru (1796) and motmot (1651), French haha 'ditch' (1712), and Maori kaka (1774). Even the nursery words mama and papa were borrowed from French in the 17th century. So-so is probably the sole native formation from the EMnE period; it is first recorded in 1530." (Wadsworth, 2012)

形态和语音

Sharon Inkelas在“重叠研究”中写道，有两种不同的方法，产生两种不同类型或子集的重叠：语音重叠和形态重叠。“下面我们列出了一些判断复制效果何时为重复以及何时为语音重复的标准。

(1) Phonological duplication serves a phonological purpose; morphological reduplication serves a morphological process (either by being a word-formation process itself or by enabling another word-formation process to take place...). (2) Phonological duplication involves a single phonological segment...; morphological reduplication involves an entire morphological constituent (affix, root, stem, word), potentially truncated to a prosodic constituent (mora, syllable, foot). (3) Phonological duplication involves, by definition, phonological identity, while morphological reduplication involves semantic, not necessarily phonological, identity. (4) Phonological duplication is local (a copied consonant is a copy of the closest consonant, for example), while morphological reduplication is not necessarily local."​ ("Morphological Doubling Theory: Evidence for Morphological Doubling in Reduplication." ed. by Bernhard Hurch. Walter de Gruyter, 2005)