"expressing reciprocal obligations," from Greek synallagmatikos, from synallagma "a covenant, contract," from syn- "together with" (see syn-) + allagma "thing taken in exchange," from stem of allassein "to exchange, barter," from allos "another," from PIE root *al- "beyond."

"junction between two nerve cells," 1899, medical Latin, from Greek synapsis "conjunction," from or related to synaptein "to clasp, join together, tie or bind together, be connected with," from syn- "together" (see syn-) + haptein "to fasten" (see apse). Introduced by English physiologist Charles Sherrington (1857-1952), summarizing recent work by other neurologists, in the 1897 revision of Sir Michael Foster's "Textbook of Physiology;" the form of the coinage is owing to the suggestion of English classical scholar Arthur Woollgar Verral (1851-1912).