means the two points where the moon's orbit crosses earth's orbital plane around the sun, and eclipses can only occur when sun, earth, and moon line up near one.

from named 'nodes' from latin nodus, meaning knot, since ancient astronomers pictured the moon's tilted orbit as literally knotting through the ecliptic; babylonian and later indian astronomers tracked these crossing points to predict eclipses centuries before anyone understood orbital mechanics.