Substance combination; Taking MDMA and 2C-B, also known as "Nexus".
MDMA + 2C-B = Nexus flipping
Substance combination; Taking MDMA and Mescaline, be it by ingesting psychedelic cacti, tea, crude extract, clean extract by means of A/B extraction, or just mescaline itself (most likely synthetic).
MDMA + Mescaline = cactus flipping
Shorthand way of writing "Phenethylamine", an alkaloid and monoamine. Phenethylamines are a family of compounds that include neurotransmitters, hormones, stimulants, 'hallucinogens' and psychedelics, entactogens and empathogens, anorectics, bronchodilators, and antidepressants.
In the chemical index section of Alexander Shulgin's book Pihkal, the word phenethylamine is replaced by the abbreviation "PEA".
Examples of PEA's: Dopamine, (Nor)epinephrine or adrenaline, various amphetamines, MDA/MDMA, Mescaline, DOx's, 2C's, and so on.