adverb, adverb taking the 'to' particle
remarkably; noticeably; considerably
Describes a clear, often sudden change in degree or state, especially a decline or increase that is easy to perceive. Commonly used with verbs like 減る, 増える, 寒くなる, 涼しくなる.
秋になると、めっきり涼しくなった。
When autumn came, it became noticeably cooler.
最近、彼の姿をめっきり見かけなくなった。
Lately, I've seen him around considerably less.