Syzygy
A straight-line alignment of three bodies, most often Sun, Earth and Moon.
Syzygy covers both configurations of the Sun–Earth–Moon system: conjunction, with the Moon between Sun and Earth at new moon, and opposition, with Earth in the middle at full moon.
Because the lunar orbit is inclined about 5° to the ecliptic, most syzygies are imperfect and nothing happens. When one falls near a node — where the two orbital planes cross — the alignment is close enough to produce an eclipse. That is why eclipses arrive in seasons rather than every month.