A weakening or loss of strength/vitality — literally wilting (as with plants) or a decline in vigor.
夏の高温で花の萎えが目立ってきた。
The flowers' wilting due to the summer heat has become noticeable.
長時間の作業で集中力の萎えを感じた。
I felt a loss of concentration after working for a long time.
水切れが続くと苗の萎えが早まる。
If the plants are deprived of water continuously, they wilt faster.
Colloquial/slang usage meaning disappointment or loss of interest/enthusiasm (e.g., 'that was a letdown; I'm not excited anymore').
その発言でみんなが萎えた。
Everyone felt let down by that remark.
景品がしょぼくてテンション萎えた。
The prizes were lame, so my enthusiasm dropped.
試合が早々に決まって、会場の盛り上がりが萎えた。
The match was decided early and the crowd's excitement died down.