sound-symbolism (n.)

A term used in SEMIOTICS and LINGUISTICS to refer to a direct association between the FORM and the MEANING of LANGUAGE: the sounds used reflect properties of the external world, as in cases of onomatopoeia (e.g. cuckoo, murmur, crash) and other forms of SYNAESTHESIA (e.g. sl- in such words as slimy, slither). Sound-symbolic words are also sometimes referred to as mimetic.