Twilight is the period between day and night when the sun is below the horizon but its light is still visible. There are three distinct twilight phases, each defined by how far the sun is below the horizon.
Civil Twilight
Civil twilight occurs when the sun is 0-6° below the horizon. During this time, there's enough natural light for most outdoor activities without artificial lighting. This is when most people consider it "dawn" or "dusk." Street lights typically turn on/off at the end of civil twilight.
Nautical Twilight
Nautical twilight occurs when the sun is 6-12° below the horizon. The horizon is still visible at sea (hence "nautical"), allowing sailors to take star sightings for navigation while still seeing the horizon. The sky takes on deep blue tones during this phase.
Astronomical Twilight
Astronomical twilight occurs when the sun is 12-18° below the horizon. The sky is dark enough for most astronomical observations, though some faint sky glow remains. True night (full darkness) begins when the sun is more than 18° below the horizon.
Blue Hour
The "blue hour" overlaps with civil and nautical twilight - it's the period when the sky takes on a deep blue color. This occurs roughly 20-40 minutes after sunset (or before sunrise) and is prized by photographers for its unique, moody lighting.