とくに思春期から大人になる中で、外見が大きくよくなること。
"He was awkward in high school, but he really glowed up by the time he started university."
She posted a side-by-side and the caption just said 'glow up' — and honestly, she wasn't lying.
— Twitter/X, widely circulated 2016–2019 (representative example of viral usage)