synaesthesia n.

word-forming element meaning "together with, jointly; alike; at the same time," also sometimes completive or intensive, from Greek syn (prep.) "with, together with, along with, in the company of," from PIE *ksun- "with" (source also of Russian so- "with, together," from Old Russian su(n)-). Assimilated to -l-, reduced to sy- before -s- and -z-, and altered to sym- before -b-, -m- and -p-. Since 1970s also with a sense of "synthetic."

Proto-Indo-European root meaning "to perceive."

It forms all or part of: aesthete; aesthetic; anesthesia; audible; audience; audio; audio-; audit; audition; auditor; auditorium; auditory; hyperaesthesia; kinesthetic; oyer; oyez; obedient; obey; paraesthesia; synaesthesia.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit avih, Avestan avish "openly, evidently;" Greek aisthanesthai "to feel;" Latin audire "to hear;" Old Church Slavonic javiti "to reveal."

word-forming element in names of countries, diseases, and flowers, from Latin and Greek -ia, noun ending, in Greek especially used in forming abstract nouns (typically of feminine gender); see -a (1). The classical suffix in its usual evolution (via French -ie) comes to Modern English as -y (as in familia/family, also -logy, -graphy). Compare -cy.

In paraphernalia, Mammalia, regalia, etc. it represents Latin or Greek -a (see -a (2)), plural suffix of nouns in -ium (Latin) or -ion (Greek), with formative or euphonic -i-.