Definitions

Sound symbolism is a linguistic phenomenon that consists of a relationship of analogy between the sound shape of words and their meaning.

Phonosemantics is a branch of Linguistics that studies the causes of sound symbolism and its applications in verbal communication.

References:

Filippo M. Leonardi, A Geometrical Approach to Phonosemantics (2013).

Filippo M. Leonardi, Phonesthemes in Latin Language (2015).

Filippo M. Leonardi, The First Principle of Phonosemantics (2016).

In linguistics, sound symbolism is the perceptual similarity between speech sounds and concept meanings. It is a form of linguistic iconicity. For example, the English word ding may sound similar to the actual sound of a bell. 

Linguistic sound may be perceived as similar to not only sounds, but also to other sensory properties, such as size, vision, touch, or smell, or abstract domains, such as emotion or value judgment. Such correspondence between linguistic sound and meaning may significantly affect the form of spoken languages.

A phememe (from Ancient Greek φημί ‘I speak, say’) is a hypothesized speech sound with an abstract gestural meaning, proposed by the linguist Mary LeCron Foster.

A phonestheme (/foʊˈnɛsθiːm/ foh-NESS-theem; phonaestheme in British English) is a pattern of sounds systematically paired with a certain meaning in a language. The concept was proposed in 1930 by British linguist J. R. Firth, who coined the term from the Greek φωνή phone, "sound", and αἴσθημα aisthema, "perception" (from αίσθάνομαι aisthanomai, "I perceive"). For example, sequence "sl-" appears in English words denoting low-friction motion, like "slide", "slick" and "sled".