is a specific kind of: period, period of time, time period.

is a broader term that covers: weeknight | wedding night.

has as a part: evening | late-night hour | midnight | small hours | lights-out.

is a part of: 24-hour interval, day, mean solar day, solar day, twenty-four hour period, twenty-four hours.

is the opposite of: day, daylight, daytime.

derivation: nightly.