1.  sunset(n = noun.time) sundown - the time in the evening at which the sun begins to fall below the horizon;
is a kind of hour, time of day
is a part of eve, even, evening, eventide
Antonym: sunrise

2.  sunset(n = noun.phenomenon) - atmospheric phenomena accompanying the daily disappearance of the sun;
is a kind of atmospheric phenomenon

3.  sunset(n = noun.event) - the daily event of the sun sinking below the horizon;
is a kind of periodic event, recurrent event

1.  sunset(s = adj.all) - of a declining industry or technology; "sunset industries"

2.  sunset(s = adj.all) - providing for termination; "a program with a sunset provision"

1.  The descent of the sun below the horizon; also, the time when the sun sets; evening. Also used figuratively. [1913 Webster]
"'T is the sunset of life gives me mystical lore." [1913 Webster]

2.  Hence, the region where the sun sets; the west. [1913 Webster]

sunset, n.
1 the sun's setting in the evening.
2 the coloured sky associated with this.
3 the time at which sunset occurs.
4 the declining period of life.

N  evening, eve, decline of day, fall of day, close of day, candlelight, candlelighting, eventide, nightfall, curfew, dusk, twilight, eleventh hour, sunset, sundown, going down of the sun, cock-shut, dewy eve, gloaming, bedtime, afternoon, postmeridian, p, m, autumn, fall, fall of the leaf, autumnal equinox, Indian summer, St. Luke's summer, St. Martin's summer, midnight, dead of night, witching hour, witching hour of night, witching time of night, winter, killing time, vespertine, autumnal, nocturnal, midnight, the outpost of advancing day, sable-vested Night, this gorgeous arch with golden worlds inlay'd.