First contact is the term applied to the moment when the partial stage of an eclipse commences. The dimming is gradual. As the eclipse progresses, small crescent shapes may be seen on the ground under trees, as gaps between the leaves project the image of the Sun like a pinhole camera.
Second contact is when totality begins. It is a dramatic moment: the temperature drops sharply and birds become quiet. The stars become visible, together with solar prominences. No partial eclipse can approach this effect.
Third contact, when totality ends, is also very dramatic as daytime, albeit rather dark, is restored in a moment.
Fourth contact is the moment when the final partial stage ends, and is always very anticlimactic.