noun The time when the Sun is below the horizon when the sky is dark.
Most animals are awake at day and sleep at night.
noun The period of darkness beginning at the end of evening astronomical twilight when the sun is 18 degrees below the horizon, and ending at the beginning of morning astronomical twilight.
noun A period of time often defined in the legal system as beginning 30 minutes after sunset, and ending 30 minutes before sunrise.
noun An evening or night spent at a particular activity.