Synaesthesia (also spelled synesthesia or synæsthesia) is the experience of a blending, merging, or mixing of the senses.  For example, a person experiencing synaesthesia may begin seeing music, tasting colors, hearing smells, or any other potential combination of the senses.  At its highest level, synaesthesia becomes so all-encompassing that each of the senses become completely intertwined with the rest of one's senses. This is a complete blending of human perception and is usually interpreted as extremely profound when experienced. It is worth noting that a significant percentage of the population experience synaesthesia to varying extents during everyday life without the use of drugs.  
Synaesthesia is commonly induced under the influence of heavy dosages of psychedelic compounds,  such as LSD, psilocybin, and mescaline. However, it is seemingly most commonly experienced under the influence of stimulating psychedelics such as the 2C-x, DOx, and Nbome series.
- Synaesthesia (Dictionary of Psychology) | https://dictionary.apa.org/synesthesia
- Why are there different types of synesthete? | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3759026/
- Synaesthesia: the prevalence of atypical cross-modal experiences (ncbi) | https://www.ncbi.nlm.nih.gov/pubmed/17076063
- Modality and variability of synesthetic experience | https://www.ncbi.nlm.nih.gov/pubmed/?term=22428428
- The induction of synaesthesia with chemical agents: a systematic review (ncbi) | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3797969/