The stage, or stages, of an eclipse, occutation, or transit when the edges of the apparent disks of astronomical bodies seem to touch.

At a solar eclipse, first contact is when the advancing edge of the sun first appears to touch the moon; the second contact is when the advancing edge of the sun seems to touch the other side of the moon, beginning totality; the third contact is when the trailing edge of the sun "touches" the trailing edge of the moon, ending totality; and the fourth contact marks the end of the eclipse.