An ideophone (also known as a mimetic or expressive) is a member of the class of words that depict sensory imagery or sensations, evoking ideas of action, sound, movement, color, or shape. The class of ideophones is the least common syntactic category cross-linguistically; it occurs mostly in African, Australian, and Amerindian languages, and sporadically elsewhere. Ideophones resemble interjections but are different owing to their special phonetic or derivational characteristics, and based on their syntactic function within the sentence. They may include sounds that deviate from the language's phonological system, imitating—often in a repetitive manner—sounds of movement, animal noises, bodily sounds, noises made by tools or machines, and the like.

While English does have ideophonic or onomatopoetic expressions, it does not contain a proper class of ideophones because any English onomatopoeic word can be included in one of the classical categories. For example, la-di-da functions as an adjective while others, such as zigzag, may function as a verb, adverb or adjective, depending on the clausal context. In the sentence "The rabbit zigzagged across the meadow", the verb zigzag takes the past -ed verb ending. In contrast, the reconstructed example *"The rabbit zigzag zigzag across the meadow" emulates an ideophone but is not idiomatic to English.

Dictionaries of languages like Japanese, Korean, Xhosa, Yoruba, and Zulu list thousands of ideophones. Sometimes ideophones are called phonosemantic to indicate that it is not a grammatical word class in the traditional sense of the word (like verb or noun), but rather a lexical class based on the special relationship between form and meaning exhibited by ideophones. In the discipline of linguistics, ideophones have sometimes been overlooked or treated as a subgroup of interjections.

The word ideophone was coined in 1935 by Clement Martyn Doke, who defined it in his Bantu Linguistic Terminology as follows.

A vivid representation of an idea in sound. A word, often onomatopoeic, which describes a predicate, qualificative or adverb in respect to manner, color, sound, smell, action, state or intensity.

Ideophones evoke sensory events. A well known instance of ideophones are onomatopoeic words—words that imitate the sound (of the event) they refer to. Some ideophones may be derived from onomatopoeic notions. In many languages, however, ideophones do not solely represent sound. For instance, in Gbaya, kpuk 'a rap on the door' may be onomatopoeic, but other ideophones depict motion and visual scenes: loɓoto-loɓoto 'large animals plodding through mud', kiláŋ-kiláŋ 'in a zigzagging motion', pɛɗɛŋ-pɛɗɛŋ 'razor sharp'.

Ideophones are often characterized as iconic or sound-symbolic words, meaning that there can be a resemblance between their form and their meaning. For instance, in West-African languages, voiced consonants and low tone in ideophones are often connected to largeness and heaviness, whereas voiceless consonants and high tones tend to relate to smallness and lightness. Reduplication figures quite prominently in ideophones, often conveying a sense of repetition or plurality present in the evoked event. The iconicity of ideophones is shown by the fact that people can guess the meanings of ideophones from various languages at a level above chance. However, the form of ideophones does not completely relate to their meaning; as conventionalized words, they contain arbitrary, language-specific phonemes just like other parts of the vocabulary.^{[citation needed]}

The grammatical function of ideophones varies by language. In some languages (e.g. Welayta, Yir-Yiront, Semai, Korean), they form a separate word class, while in others, they occur across a number of different word classes (e.g. Mundang, Ewe, Sotho, Hausa).