The period of light between dawn and nightfall; the interval from sunrise to sunset. The 24-hour period during which the earth completes one rotation on its axis. The period during which a celestial body makes a similar rotation. One of the numbered 24-hour periods into which a week, month, or year is divided. The portion of a 24-hour period that is devoted to work, school, or business: an eight-hour day; a sale that lasted for three days. A 24-hour period or a portion of it that is reserved for a certain activity: a day of rest. A specific, characteristic period in one's lifetime: In Grandmother's day, skirts were long. A period of opportunity or prominence: Every defendant is entitled to a day in court. That child will have her day. A period of time in history; an era: We studied the tactics used in Napoleon's day. The day of computer science is well upon us. Period of life or activity: The sick cat's days will soon be over. Of or relating to the day. Working during the day: the day nurse. Occurring before nightfall: a day hike. day after day For many days; continuously. day in, day out Every day without fail; continuously.