1
to be clear (of a sight, sound, colour, etc.);to be bright;to be vivid;to be crisp
満月が空にさえている。
A full moon is shining bright in the sky.
2
to be clear-headed;to be alert;to be on the ball;to be wide awake
昨日ゆっくり休んだ分、今日は頭が冴えている。
I feel smart today, to a degree that is in proportion to the amount of good rest I had yesterday.
3
to look upbeat;to be perky;to be cheerful
often as 冴えない
4
to master (a skill);to excel at;to cleanly execute
5
6
to become frigid;to become intensely cold
original meaning; now somewhat old-fashioned
Other forms
冱える 【さえる】 (rarely used kanji form)