c.1400, "body of attendants; also "meeting of armed forces" (mid-15c.); main modern sense of "coming together of people, a meeting" is from 1520s; from Latin congressus "a friendly meeting; a hostile encounter," past participle of congredi "meet with, fight with," from com- "together" (see com-) + gradi "to walk," from gradus "a step" (see grade (n.)).
Sense of "meeting of delegates" is first recorded 1670s. Meaning "sexual union" is from 1580s. Used in reference to the national legislative body of the American states since 1775 (though since 1765 in America as a name for proposed bodies). Congress of Vienna met Nov. 1, 1814, to June 8, 1815, and redrew the map of Europe with an eye to creating a balance of powers after the disruptions of Napoleon.