1to become tight or firm; to become toned or tense — used for muscles, expressions, atmosphere, etc.
毎朝のトレーニングで体が少しずつ引き締まってきた。
With daily training, my body has gradually become firmer.
試験当日、教室の空気が引き締まった。
On the exam day, the atmosphere in the classroom became tense.
腹筋を鍛えたらウエストが引き締まった気がする。
After strengthening my abs, I feel my waist has tightened.