A short explanation

Synaesthesia can be explained as "combined sensation", which is just a translation of "syn" (Greek: together) and "esthesia" (from "aisthesis", Greek: sensation). This "combined sensation" in synaesthesia means that when a sense is activated, another unrelated part of the same sense or also another sense is activated concurrently. Let's have a look at an example: when synaesthetes hear a sound or see a digit, they immediately see a (synaesthetic) colour, either in "their mind's eye" or directly on the written digit. They might also experience synaesthetic forms, tastes, smells, sounds, touch etc. These synaesthesiae can also be influenced or even elicited by emotions or by cognitive inducers (e.g., the concept of 12 months forming one year).

Synaesthesia is naturally occurring and - important to note! - it is not a disease. Synaesthesia-like phenomena might also be experienced in the course of some illnesses, wounds or drug use. But in contrast to this, the naturally occurring synaesthesia has nothing to do with diseases, hallucinations or the like. Furthermore, scientific studies have shown that many synaesthetes perform better on tests of memory and colour discrimination. Synaesthesia cannot be learned and is very stable through life. It is assumed to be hereditary even though the exact genetic mechanisms are not known yet.